It is not possible to downgrade an iOS version on your iPhone. You can only update. Also, currently, there is iOS version 15.7 and iOS version 16.0.3 available to update.
I want to let you know that Apple does NOT force people to update their iPhones. No one has to update if they don’t want to. If an app influenced your decision to update, then the app required a higher iOS version to run and that is decided by the developer of the app and not Apple. As technology and performance improve, then there will be updates to match, however, no one is forced to update. Just turn OFF Automatic Updates feature on the iPhone.
By the way, there are some very good improvements over iOS 14 and iOS 15 in iOS version 16.0.3. I have updated to the latest and am not experiencing any issues. Again, you don’t have to and I am just sounding my opinion.
FYI: These are user-only forums. This is not Apple Support. if you need to contact Apple Support, then contact product and tech support: 👉 Contacting Apple for support and service including international calling numbers.
Axel F.