How do i update my iPhone 6

Your iPhone 6 can be updated to 12.5.1, no further.

Update your iPhone, iPad, or iPod touch - Apple Support

issues?

Updating iOS: common issues and solutions - Apple Community

video..

The iPhone 6 can be updated to iOS 12.5 or so, and no further.

iPhone 6 lacks the memory and processor hardware needed for newer iOS versions.

The upgrade from iPhone 6 to iPhone 6s doubled the memory, as well as a better processor.

There is no hardware upgrade available, short of a newer or new iPhone.