iOS Update IPhone 6 Update version iOS 14.0 or later

Hello,

There are different ways to ensure the highest iOS that your device will support is installed on your device.

This support article has instructions about updating your iPhone or iPad wirelessly --> Update your iPhone or iPad - Apple Support

You can also use Mac and PC computers to update system software: Update iOS on iPhone - Apple Support

When updating, your device will update to the highest version currently available that it will support, or it will just say it is up to date and can't install a higher version. An iPhone 6 will update to iOS 12.5.7 Apple indicates ( Unable to update my iPhone 6 Plus from 12… - Apple Community ) I need to point out the obvious and state that if you want to run a higher system version than your current device will run, that you need to buy a newer device.

An iPhone 6 is not capable of updating to anything higher than iOS 12.5.7. It cannot run iOS 13, 14, 15 or 16.