Software update

Hello ~ Your iPhone is on the highest version of iOS supported by your current device. If you require or want to run a higher version of iOS you need a newer device. You can go to the black line at the top of this page and click on iPhone to see the newest options.

Also:

Apple - Official Site

If you choose to purchase a second hand device please take time to read through

The All Too Common SAD Reality of Buying … - Apple Community

~Katana-San~

The highest version of iOS you can run on an iPhone 6 is iOS 12.5.5. If you are looking for anything higher, iPhone 6 release almost 8 years ago is lacking the hardware required to run the newer iOS versions. If you need the newer iOS versions, then you'll need to purchase a newer, more modern iPhone at this point.

An iPhone 6 cannot update past iOS 12.5.5. You need a new phone.