Can’t get any update

Hello. Your iPhone 6 Plus is an older device and is unable to run any version of iOS later than 12.5.5. If you would like to run iOS 15, you will need to upgrade to a newer iPhone. These iPhone models are compatible with iOS 15.

Hello ~ Your iPhone 6 Plus is not capable of being updated to iOS 15. If you need/require a higher version of iOS you will need a newer device. You can go to the black line at the top of this page and click on iPhone to see new choices.

Or:

Apple Store Online

Also if you decide to buy a second hand device…please read this first:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

~Katana-San~

The iPhone 6/6 Plus do not have the required hardware to update to iOS 13 or higher.