On the other device, open a web browser and go to iCloud.com/find
Sign in with the Apple ID used on the iPhone
Select the iPhone from the All Devices list
Click Remove from Account if the iPhone is offline or already erased
If the iPhone is online, choose Erase iPhone first
After the erase is complete, click Remove from Account
On a Mac, open the Find My app
Click Devices
Select the iPhone
Click Remove This Device
If prompted, enter the Apple ID password
If you do not know the Apple ID password, reset it at iforgot.apple.com
If Activation Lock is enabled, turn off Find My iPhone only by signing out of iCloud on the iPhone or by removing the device from the Apple ID account
If the iPhone is still in use by someone else, ask the owner to remove it from their account