Hello,
Usually, the default PIN is "0000" or "1234". You may also be able to find the PIN on the accessory itself or check the documentation for the accessory. Note that when you’re pairing a Bluetooth keyboard with your phone, you’ll often be prompted to enter a passcode on the keyboard itself to make sure you’re pairing with the correct device. When you’re done typing the passcode on the keyboard, make sure you press Enter on the keyboard.
If you're trying to restart the pairing process of your Bluetooth device, go to Settings > Devices > Bluetooth & other devices and select the device you want to remove, then select Remove device.
For more information on how to fix Bluetooth problems in Windows 10 Mobile, you can refer to this article.
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