1st Step : Sign in to your iCloud Account.
2nd Step : Click the “All Devices” dropdown at the top of the screen.
3rd Step : click on the device you would like to remove. A grey “X” will appear next to the device.
4th Step : Click on the grey “X” and then select “Remove” from the next window.
5th Step : A confirmation window will show that the device has been removed.
Please Note In addition, you can reference Remove iCloud Lock for a Step-by-Step video tutorial on how to unlink your device from your iCloud account researching youtube .