how to change the Reference Prefix of an existed part

in the "New Part Property" mask, we can input the letter for the "Part Reference Prefix". But as soon as the new part is defined, it seemed I could not change the prefix letter later.

 I tried somehow but coundn't get the solution except reopenning a new part properties mask. (but it is inefficient)

Can anybody here do me a favor?

  am using OrCAD 16.6