well you guys are close, but the part to sharpen is not the punch, which is the part that threads on, but the die, and you can use a belt sander..... if you can hold it perfectly flat, but I have used a shop grinder with a magnetic table, of course if the punch is banged up a bit you can use a file to only dress up the edges, but the cutting is done by the die.
you do not want to make the punch any smaller than it is !
