If the you are unable to access the device, Find My iPhone can be disabled online via iCloud.
To disable Find My iPhone, the device must have a valid network connection to either Cellular Data or Wi-Fi. Without a connection, Find My iPhone is disabled on the device but not from Apple's servers.
From the Home screen, navigate: Settings > iCloud.
Tap Find My iPhone (scroll to the bottom).
Tap the Find My iPhone switch to turn on or off .
If prompted, enter your Apple ID password then tap Turn Off.
If desired, tap the Send Last Location switch to turn on or off.