I have an old PO box (Grecian, I think) with a combination lock that uses the letters from A to J. I know the current combination but would like to change it.

When I take the back off I see three discs, each with holes around the circumference and a post in one hole on each disk. I'm guessing that I need move the posts to the hole that corresponds to the letter I want...but not sure how to do this. Any help would be appreciated. Thanks.