www.iCloud.com website.
Log into icloud.com/find with Apple ID and password
Select device from “All Devices” drop down and select your device (top, centre of screen)
Device pop up box will appear.
Click “Remove from Account “
confirm by clicking “Remove”
B: From another Apple device with same Apple ID (iOS 10.3 or later)
Tap Settings > [your name], then scroll down.
Tap any device name to view that device's information
Scroll down and Tap [ Remove from Account ]