iOS updates

iOS 14 is only supported on the iPhone 6s and newer. The latest software for the iPhone 6 Plus is iOS 12. Apple still regularly releases security updates for these devices, as shown on this list: Apple security updates - Apple Support

As of now, the latest software for your device is 12.5.6

This link has information on that update: About the security content of iOS 12.5.6 - Apple Support

I hope this helps!

That is true; an 8 1/2 year old iPhone 6 hardware is incapable of running a version higher than 12.5.6. If you need a newer version you need a new phone.

The 6 Plus is not compatible with iOS 13 or higher.

The highest possible for that model is iOS 12.5.6.