How do I update if it’s not showing the iOS update

Hello ~The reason for that is that your iPhone 6 cannot be update to a higher version of iOS. If you need/ want to run a higher version of iOS you need a newer iPhone. Go to upper black line and click on iPhone to see your new choices. If you are considering a second hand device please take the time to read this information:

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~

The iPhone 6 cannot be updated beyond iOS 12.

~Lyssa