How to update iPhone 6 iOS

Sorry, but the iPhone 6 can only be updated to iOS 12.5.6

You will need a newer model to be able to install newer iOS versions.

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

iOS 14 and iPadOS are compatible with these devices - Apple Support

iOS 15 and iPadOS 15 are compatible with these devices - Apple Support

Unfortunately an iPhone 6 cannot support iOS versions beyond 12.5.6

See why in this compatibility chart:

An iPhone 6/6plus can't be updated beyond 12.5.6, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.