I want to update my iPhone

Hello ~ If you have an iPhone 6 as listed you may only update to iOS 12.5.6. If you require a higher version of iOS you need a newer iPhone. You may go to upper left and click on 2 lines to see new choices but if you choose a second hand device please take a moment to read these…

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~