When you are signed in to iCloud.com and click “Find my iPhone”, you will open a list of devices that are linked to your account. Select the device that you want to remove from your account. Click “Remove from account”.
How do I delete a locked Apple ID?
To add a device to your account: go to icloud.com, select the device you want to add and then go to “Find my iPhone/iPad”.Select “All Devices” to open a list of devices linked to your account, select the device to be added, click “add” and then click “done”.