How do I update iOS on my iPhone 7?

What iOS are you trying to update your iPhone 7 to? It cannot be updated higher than iOS 15.8.3, and no longer receives the latest iOS software updates. If you are trying to update to that, you can do so in Settings > General > Software Update.

If you mean updating your iPhone higher than iOS 15.8.3, there are no instructions anyone can send you. Your over 8 year old iPhone is not capable of being updated higher than iOS 15.8.3. If you need a higher level iOS version, then you'll have to retire your iPhone and buy a new or much newer model iPhone.

No.

User to user support forum.

iPhone 7 can’t be updated past iOS 15.

iPhone 7 cannot be updated to iOS 16, iOS 17 or iOS 18......if that is what you are asking.

If you really want to update to current iOS versions......you need a new phone.