Want to iPhone 6 update iOS 15

An iPhone 6 can't be updated beyond 12.5.7, it doesn't have the hardware required to run anything higher.

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

What you want is unfortunately not going to happen.

The last update for the iPhone 6 was iOS 12.5.7. No more updates are coming.

If you want to run iOS 15, you will need at least an iPhone 7 or higher model.

Unfortunately, that is not possible.

The iPhone 6 can be updated to 12.5.7, no further.