I purchased a replacement PTAC that has a 5-20P. It turns out that the original machine had a 6-30P. Now I have a 5-20P and a 6-30R outlet. Is it possible to change the 6-30R to a 5-20R? What is the process?

A 6-30 is a 240V/30A.
A 5-20 is a 120V/20A

Since you are going down in amperage you can do this, as long as the 6-30R is on a dedicated circuit.
You have to change the receptacle, and change the two-pole 30A breaker for a single-pole 20A. You then have to take the white wire and move it to the neutral bar in the panel.