This article explains how to turn off Find My iPhone with or without the phone itself.

If you're ready to sell or give away your old iPhone, it's a good idea to turn off Find My, erase your iPhone's data, and sign out of iCloud. If you're worried that someone is using your iPhone to track you but you want to keep using it, turn off Find My and skip those other steps.

Tap your name at the top.

Enter your password and tap Disable.