How do i upgrade from 12.55 to 14iso

For an iPhone 6 Plus, you can not. That model can be updated to iOS 12.5.7, no further.

An iPhone 5s or iPhone 6 isn't capable of 13 or above; they require at least 2GB of RAM. To install 12.5.7, use a computer or the Software Update function in Settings.

(240503)