Why my iPhone 6 Plus it’s not updating to iOS 15

Hello ~The 6 plus cannot be updated to iOS 15. In order to use a higher version of iOS you will need a newer iPhone. You can go to upper black line to see the newest choices. If you consider 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 last model to receive the iOS 15 update is iPhone 6S. Your phone doesn't support the update.

The iPhone 6 series was supported by iOS 8-12. 