My iPhone 6 not update iOS 14

The only way to solve the problem is to retire you nearly 10 year old iPhone and purchase a newer, more modern iPhone. iPhone 6 is NOT capable of being updated past iOS 12.5.x. It doesn't have the hardware required and it never will be able to update further than iOS 12.

The iPhone 6 cannot install anything higher than iOS 12.5.7. Frankly, you need a new phone.

iOS 14 is no longer available, but even if it was, you would not be able to install it on your iPhone 6.

If you have an iPhone 7, you can install iOS 15. If you have an iPhone 8, you can install iOS 16. If you have an iPhone XR or XS or higher model, you can install iOS 17.

You don't.

An iPhone 6 can't be updated beyond 12.5.7, 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.

Nothing to solve.

The iPhone 6 is not compatible with iOS versions greater tan 12.5.7.

You cannot install iOS 14 on an iPhone 6, as it lacks the hardware, RAM and CPU for to properly run that version.