ios update iphone 6 plus

You can't.

iOS 12.5.4 is the last version for an iPhone 6 Plus.

Hello. The iPhone 6 Plus cannot run any versions of iOS later than 12.5.4. If you are already running this version, you will need to upgrade to a newer iPhone to run a later version of iOS.

If you are on a version older than iOS 12.5.4 and would like to upgrade to the newest release for your iPhone 6 Plus, you can do so by following the steps here.

Iphone 6 cannot run any version of iOS past iOS 12.

It doesn't have enough RAM.