Please help me upgrade my iPhone to iOS 14.0

Hello ~ Your iPhone 6 cannot be upgraded any further. You need a newer device. Go to upper left and click on 2 lines the 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~

That’s not possible. Your phone is not compatible with anything later than iOS 12.5.5.