Why won’t my phone up date it’s stuck in iOS 12.5.5 and it shows no up date and as of today we are on iOS 15.4

The iPhone 6 does not have the required hardware to update to iOS 13 or higher.

It can be updated to iOS 12.5.5, no further.

Your iPhone 6 cannot run any newer software than iOS 12.5.5. It lacks the components to run newer software. If you want a newer version, you will need to buy a newer phone. Technology marches on!

An iPhone 6 or 6 Plus CANNOT be upgraded past iOS 12.5.5.