How to update my iPhone 6 Plus

You can't, if you are already running iOS 12.5.6, since that is as far as the iPhone 6 Plus can go, update wise.

First go to Settings > General > About > Version If it says iOS 12.5.6 that is as high as you can go. The iPhone 6 Plus does not have sufficient RAM to run iOS 13 or later and you cannot add RAM.

If it shows a lower version go to Settings > General > Software Update