How to update my iPhone 6?

Go to Settings/General/Software Update and it will tell you what it can be updated to. For an iPhone 6, your 8 year old phone tops out at 12.5.6.

You have an iPhone 6 and iOS version 12. That is the highest iOS version that you can install on an iPhone 6. You will need a newer model iPhone to update to higher iOS versions.

Axel F.