iOS update

An iPhone 6/6 plus 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.

You need an iPhone 8 or higher model to run current software......iOS 16.

If you want to run iOS 17 in the fall when it is released, you will need an XS, XR, or higher model.