How can I upgrade my iPhone 6 Plus version

You do that (presumably you meant iOS version) by buying a new phone. The iPhone 6/6 Plus cannot be updated past iOS 12.5.5 (which was a special iOS release exclusively for the iPhone 5S/iPhone 6/iPhone 6 Plus, iPod touch 6, original iPad Air, iPad mini 2nd and 3rd generation, as these cannot be updated to iOS15 due to lack of RAM (iOS versions newer than iOS12 require a device with 2GB RAM).

If you need to use apps which require a more recent version of iOS, you will need to purchase a new phone. Most - if not all - developers have now dropped support for 1GB RAM devices. Your phone is almost 7½ years old, Apple can't support old(er) devices indefinitely.

Sorry, but the hardware of the iPhone 6Plus can't be upgraded to support a higher iOS version than iOS 12.5.5

You'll have to have at least an iPhone 6s, to be able to update the iOS any further.