how to turn off find my iphone

The phone is off line? Broken? Why can't you log into iCloud.com/Find My Phone using a computer browser? You can do the below so that if the phone comes on line, it will have Find My Phone and your data removed/erased.

Find My Phone – Remove device.    

See these documents:

See Set up Find My iPhone https://support.apple.com/en-us/HT205362

You need to have turned on Find My iPhone from this point on.

If your iPhone, iPad, or iPod touch is lost or stolen  https://support.apple.com/en-us/HT201472

Use Lost Mode in Find My iPhone on iCloud.com https://support.apple.com/guide/icloud/use-lost-mode-mmfc0f0165/icloud

If Find My iPhone is offline or not working https://support.apple.com/en-us/HT204233