• For this example I have a controlled room with a single BLE device enable.
• Your class should extend CBCentralManagerDelegate.
• Implement the method: centralManagerDidUpdateState(_ central: CBCentralManager).
• Use global queue to not freeze the screen while searching for a device.
• Instantiate CBCentralManager and wait for callback centralManagerDidUpdateState response.

class BLEController: CBCentralManagerDelegate{

var cb_manager: CBCentralManager!
var bles : [CBPeripheral] = []

    override func viewDidLoad() {
        super.viewDidLoad()
        cb_manager = CBCentralManager(delegate: self, queue: DispatchQueue.global())
    }

    func centralManagerDidUpdateState(_ central: CBCentralManager) {
        print("UPDATE STATE - \(central)")
    }
}

Callback to centralManagerDidUpdateState indicates that CoreBluetooth is ready, so you can search for BLE now. Update centralManagerDidUpdateState code to search for all BLE device when it is ready.

func centralManagerDidUpdateState(_ central: CBCentralManager) {
    print("UPDATE STATE - \(central)")
    SearchBLE()
}

func SearchBLE(){
    cb_manager.scanForPeripherals(withServices: nil, options: nil)
    StopSearchBLE()
}

func StopSearchBLE() {
    let when = DispatchTime.now() + 5 // change 5 to desired number of seconds
    DispatchQueue.main.asyncAfter(deadline: when) {
        self.cb_manager.stopScan()
    }
}

• SearchBLE() search for BLE devices and stop searching after 5s
• cb_manager.scanForPeripherals(withServices: nil, options: nil) looks for every BLE in range with you.
• StopSearchBLE() will stop the search after 5s.
• Each BLE found will callback func centralManager(_ central: CBCentralManager, didDiscover peripheral: CBPeripheral, advertisementData: [String : Any], rssi RSSI: NSNumber)

func centralManager(_ central: CBCentralManager, didDiscover peripheral:                                                             
    CBPeripheral, advertisementData: [String : Any], rssi RSSI: NSNumber) {
    guard let name = peripheral.name else {
        return
    }
    print(name)
    bles.append(peripheral)
}

• I'm in a controlled room with a single minew beacon that use IBEACON protocol.
• BLEController needs to extend CBPeripheralDelegate
• I'll use the first BLE to connect after the search has stop.
• Modify the method StopSearchBLE()

class BLEController: CBCentralManagerDelegate, CBPeripheralDelegate{
//...
    func StopSearchMiniewBeacon() {
        let when = DispatchTime.now() + 5 // change 2 to desired number of seconds
        DispatchQueue.main.asyncAfter(deadline: when) {
            self.cb_manager.stopScan()
            self.cb_manager.connect(bles.first)
        }
    }
/...
}

• In the documention of your BLE device, you should look for the SERVICE UUID and MAJOR UUID CHARACTERISTIC

var service_uuid =  CBUUID(string: "0000fff0-0000-1000-8000-00805f9b34fb")
var major_uuid =  CBUUID(string: "0000fff2-0000-1000-8000-00805f9b34fb")
func centralManager(_ central: CBCentralManager, didConnect peripheral:             
CBPeripheral) {
    peripheral.delegate = self
    peripheral.discoverServices([service_uuid])
}

func peripheral(_ peripheral: CBPeripheral, didDiscoverServices error: Error?) {
    print("Service: \(service)\n error: \(error)")
    peripheral.discoverCharacteristics([major_uuid], for: (peripheral.services?[0])!)
}

• Create a variable 'service_uuid' and 'major_uuid' like code above. '-0000-1000-8000-00805f9b34fb' is part of the standard. 'fff0' is my SERVICE UUID, 'fff2' is my MAJOR UUID characteristic and '0000' are required to fill the 4 bytes uuid 1º block.
• discoverCharacteristics([major_uuid], for: (peripheral.services?[0])!) will get major characteristic from my device gatt server and it will have NIL as value for now.
• (peripheral.services?[0])! - 0 beacuse will return a single value once I did peripheral.discoverServices([service_uuid])

func peripheral(_ peripheral: CBPeripheral, didDiscoverCharacteristicsFor service: CBService, error: Error?) {
    for characteristic in service.characteristics! {
        print("Characteristic: \(characteristic)\n error: \(error)")
        if(characteristic.uuid.uuidString == "FFF2"){
            peripheral.readValue(for: characteristic)
        }
    }
}

func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
    print("Characteristic read: \(characteristic)\n error: \(error)")
    let major = UInt16.init(bigEndian: UInt16(data: characteristic.value!)!)
    print("major: \(major)")
}

• Characteristic value will only be readable after call peripheral.readValue(for: characteristic)
• readValue will result in func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) with value in Data type.

• You need discover the services and characteristic
• You don't need read value from the characteristic before writing over it.
• will continue for, for this example, after read value. Modify func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?)
• Add a variable new_major and reset_characteristic

var reset_characteristic : CBCharacteristic!
func peripheral(_ peripheral: CBPeripheral, didDiscoverCharacteristicsFor service: CBService, error: Error?) {
    for characteristic in service.characteristics! {
        print("Characteristic: \(characteristic)\n error: \(error)")
        if(characteristic.uuid.uuidString == "FFF2"){
            peripheral.readValue(for: characteristic)
        }
        if(characteristic.uuid.uuidString == "FFFF"){
            reset_characteristic = characteristic
        }
    }
}
let new_major : UInt16 = 100
func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
    print("Characteristic read: \(characteristic)\n error: \(error)")
    let major = UInt16.init(bigEndian: UInt16(data: characteristic.value!)!)
    print("major: \(major)")
    peripheral.writeValue(new_major.data, for: characteristic, type: CBCharacteristicWriteType.withResponse)
}

• iPhone by deafult will send and receive bytes in Little Endian format, but my device MINEW witch chipset NRF51822 have ARM archteture and need bytes in Big Endian format, so I have to swap it.
• BLE Device documentation will say what type of input and output each characteristic will have and if you can read it like above (CBCharacteristicWriteType.withResponse).

func peripheral(_ peripheral: CBPeripheral, didWriteValueFor characteristic: CBCharacteristic, error: Error?) {
    print("Characteristic write: \(characteristic)\n error: \(error)")
    if(characteristic.uuid.uuidString == "FFF2"){
            print("Resetting")
            peripheral.writeValue("minew123".data(using: String.Encoding.utf8)!, for: reset_characteristic, type: CBCharacteristicWriteType.withResponse)
        }
    if(characteristic.uuid.uuidString == "FFFF"){
        print("Reboot finish")
        cb_manager.cancelPeripheralConnection(peripheral)
    }
}

• To update a gatt server information you have to reboot it programmatically or save data to it and turn off and turn on manually.
• FFFF is characteristic that do it in this device.
• 'minew123' is the default password for reboot o save information in this case.
• run your app and watch you console for any error, I hope none, but you will not see the new value yet.

func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
    print("Characteristic read: \(characteristic)\n error: \(error)")
    let major = UInt16.init(bigEndian: UInt16(data: characteristic.value!)!)
    print("major: \(major)")
    //peripheral.writeValue(new_major.data, for: characteristic, type: CBCharacteristicWriteType.withResponse)

}

• Last step is to comment last line in method didUpdateValueFor and rerun the app, now you will the new value.