How to upgrade iOS

Sorry, this is not possible.

The best that the iPhone 6 can do is iOS 12.5.7

If you want to run iOS 16, you will need a new phone.

The iPhone 6 cannot be updated beyond iOS 12. It lacks the components to run newer software. to run newer software you will need a newer iPhone.

Damilarelawa94 Said:

"How to upgrade iOS 16.5 on iPhone 6"

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Updating an iPhone 6 Past iOS iOS 16.5:

A. You can't.  The iPhone 6 will only install to iOS 12. See here: Update your iPhone or iPad Software if you're Experiencing Issues with Location, Date, and Time - Apple Support. And, for the note, even if you could update, only the latest version will install: iOS 16.

So, what to Try:

B. Update Using a Computer:

Have a Computer Handy? If so, try using a computer to update. Go here: Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough.