how to turn of call forwarding?

The same way you would have turned it on: Settings->Phone->Call Forwarding

It depends on your Carrier because Call forwarding feature is Carrier Specific.

Watch the GIF below as an example; made on iPhone 6 iOS 12.4.4 using Airtel India Carrier Service

Besides what Phil has replied, beware that Call Forwarding is a carrier-based feature. It has to be activated on your number (via the SIM card) for it to be available.