I want to update my iPhone 6 iOS

Marking your own post as Solved may prevent others from trying to help because they think you have fixed your problem.

An iPhone 6 can't be upgraded past IOS 12.5.6.

See the article(s) below. 

iOS 16 compatible devices

IOS 15 and iPadOS 15 Compatible Devices.  *

iOS 14 and iPadOS 14 Compatible Devices.  *

iOS 13 and iPadOS are compatible with these devices - Apple Support.  *