How do I turn off find my phone
How do I turn off find my phone from another device
iPhone 6s, iOS 13
Why do you want to? If your iPhone is stolen or missing if you turn it off you will no longer be able to find it and someone else can use it.
If on the other hand you sold it and forgot to turn it off: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support