You should be able to do that online if you're not near the post office that keeps your mail. You may have to cancel the first hold mail, then do a second one. You might want to call them and ask that.
There is a phone number you can call for that I betcha. I called once to check out why I had not gotten a package. It is a 800 number and I feel you could possibly call it from anywhere in the states, possibly outside the states. Bet you can look for uspo wherever you are or the internet.