apple id

On the Device: Settings > [Tap your Name] > Scroll Down < Sign Out

You cannot sign out remotely

You cannot sign out using another device.

You can use the computer:

1. Sign in to iCloud.com with their Apple ID.
2. Go to Find My iPhone.
3. Click All Devices at the top of the screen.
4. Select the device that you want to remove from iCloud.
5. If necessary, click Erase [device].
6. Click Remove from Account.