Neil Gonzalez, writing for Wonder How To:

You can deactivate Find My iPhone on the device by going to Settings -> iCloud -> Find My iPhone and toggling it off. Alternatively, you can also just turn off iCloud altogether.

However, if you don’t have the iPhone anymore or can’t access it because of a lost passcode, the new user won’t be able to activate the iPhone, you won’t be able to get back in, or you might not be able to restore to a previous iTunes backup. Luckily, there’s a way to disable Find My iPhone remotely from a computer or other iOS device.

Read through it, then tuck this one away.