How can I update my iPhone version

Hello ~ Your iPhone 6 cannot be updated any further. You need a newer device. Go to upper black line and click on two lines in upper left….then iPhone to see your new choices. If you consider 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~

If you wish to update your iPhone 6 - given in your profile above - then you will be disappointed. The iPhone 6 cannot be updated beyond iOS 12.5.5