Can / how can i update iphone 6 plus to ios 14?

Unfortunately, the iPhone 6 is not compatible with iOS 13 or higher. That model can be updated to iOS 12.5.7.

I keep seeing threads that say that the iphone 6 plus can do it

Those threads are not correct. iOS 12.5.7 is the end of the road for the iPhone 6.

gloriale2001 wrote:

I keep seeing threads that say that the iphone 6 plus can do it

The iPhone 6S can be updated to iOS 15. The iPhone 6+ is limited to iOS 12.5.7.