Can I update my iPhone 6 from iOS 12.5.7 to iOS 15?

Sorry, but iOS 12.5.7 is as far as the iPhone 6 can go. There are no more updates for the iPhone 6.

You will need an iPhone 7 to run iOS 15 or a better choice would be an iPhone 8 to run iOS 16.

Even better would be an iPhone XR or higher model, which will run the current iOS 17 and future iOS 18.

Nope. That requires an iPhone 6S or newer. Your phone is not compatible with anything later than iOS 12.

i can’t update