How do I update iPhone 6 Plus

Go to settings and click General->Software Update, and it should show what version of iOS do you have, and if there is an update clik the install button.

An iPhone 6 Plus cannot update past iOS 12.5.5.

Because you have iphone 6 plus, you can only install security updates

Bobby_Knobloch, your first responder, gave you very good advice. Try following his directions, and let him know if you had success.

Write back to Bobby or the Community if you run into any problems. Good luck!