Open a web browser and go to iCloud.com
Sign in with your Apple ID
Select Find Devices
Click All Devices
Choose the iPhone you want to remove
Click Remove from Account if available
Enter your Apple ID password if prompted
If the iPhone is offline, click Remove from Account after it appears
If the iPhone is online, turn off Find My iPhone on the device first
On the iPhone, go to Settings
Tap your name
Tap Find My
Tap Find My iPhone
Turn off Find My iPhone
Enter your Apple ID password
Tap Turn Off