How to I upgrade iPhone 6 Plus to iOS 13.

And to answer your how-to-upgrade question…

You will need an iPhone 6s or newer, or iOS 13 and iOS 14.

There are no hardware upgrades.

Any recent or new iPhone will be substantially more capable and faster than an iPhone 6.

The iPhone 6/6 Plus can be updated to iOS 12.5.4, no further.