How do I upgrade my iPhone XS Max from 3G to either 4G or 5G?

Your XS Max out of the box supports 4G and LTE. You don’t need to do anything to use either, as long as your cellular network provider supports them.

Did you read my previous reply? You don’t have to do anything to get 4G or LTE. Your phone “knows” how to use 4G and LTE.

You may have to go to Settings/Cellular (or Settings/Mobile Data) and turn on the switch for 4G or LTE (they are the same thing, actually) but most likely it is already on.

Not sure how about