I want to upgrade my phone to iOS 15

You have an iPhone 6 Plus and it can only be updated to iOS version 12.5.6 and nothing further. You will need a newer model iPhone to download and install iOS 15/16.

Axel F.

The iPhone 6 Plus can only go as far as iOS 12.5.6. It does not have the horsepower to run anything higher.

An iPhone 7 can go to iOS 15.7

An iPhone 8 and higher can be updated to iOS 16