Not able to update

Just to clarify:

Iphone model 6s or os 6s Plus CAN be updated to iOS 15.7 version.

Iphone model 6 or 6 Plus CANNOT be updated past iOS 12.5.6 version, because of hardware (CPU/RAM) incompatibility.

Please identify your iPhone model correctly:--> Identify your iPhone model - Apple Support

Thanks and regards

Giulio

What happens when you attempt to update?

Any error messages?

What is the existing version of iOS?

I just recently find out that older phones like mine 6s plus is no longer compatible for updates such as iOS 15.7 & up.