How to update iPhone 6 Plus to iOS 14?

Unfortunately no.

Your iPhone 6+ is only capable of running iOS 12.

(Current version: 12.5.7)

If a 3rd party application requires an iOS newer that 12.x (and many do), your only option is to buy a newer phone capable of running the newer iOS.

Yes, you can download apps,

as long as the developer hasckeft older versions compatible with iOS 12 available.