I have used a mill-bastard file before to dress the edges on the standard KO cutters, but if yours are totally flattened you made need to look into professional sharpening or else maybe just replace the KO cutter itself, as the 2" isn't that expensive. I read somewhere that some guys sharpen theirs with a belt sander. That sounds like it may be a little crude, but if yours is really that bad it may not hurt to try.
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 !
A linisher with a fine belt on it would be the best thing to "sharpen" KO punches. Bear in mind, that you can wreck a decent metal tool by over-heating it, making it soft (ie: ruining the temper on it). Have water on hand and keep the tool cool, doing a wee bit at a time. And putting the wrong angle on cutting surfaces, will render a tool useless, if you don't know what them angles are, it's best to get professional folk to do the work for you.
You wouldn't get an Architect to do your electrical work, so on the same side of the coin, electricians are not tool sharpeners.
I'm lucky I suppose, I had a background in mechanical engineering/fitting before I was ever an electrician, sharpening drill-bits and what-ever is second nature to me.
Save your money and get a pro to do it, that is what they are good at.