Remove the trimmer head from shaft

The entire trimmer assembly (from the gearbox forward) will slide off of the drive shaft cover. On the side of the aluminum gearbox assembly, you will find a single screw. This is the "anti-rotation screw" (do not lose this). Then remove the small (approximately 1" diameter) clamp from the end of the gear box assembly and simply pull the entire cutting assembly off of the shaft.

If you just want to remove the trimmer bump head assembly, then you will need to lock the bump head assembly to prevent rotation while you are working. If you purchased this new, then there should be a small metal rod included with your accessories. This will fit in a small hole in the base of the gear box assembly 791-182193 and prevents the trimmer head from rotating.

Then proceed to remove the following items:

791-180814B (replaces 791-180814) Bump Head Knob Assembly
791-147495 Inner Reel (hold the trimmer string)
791-610636B (replaces 791-610636) Spring
791-612026 Retainer (be careful removing this so you do not inadvertently destroy it). If you intend to reuse the trimmer head. If not, then hack, pry, stab, etc.. until you mangle it enough to remove.
791-683301 Outer Spool & Eyelet Assembly should now slide out leaving only 791-612483
Spool Shaft which I will assume that you want to keep.

I replaced my entire string head assembly a couple of years ago (and subsequently misplaced it and am in fact wanting to purchase one) and replaced it with a grass-gater or similar.

I hope this helps.

I have been unable to find a way to remove the trimmer head so that I can replace it with an aftermarket. I've disassembled it to this point, you can see that there is no hole in the gear box to "unscrew" the head. You can see I've taken out most of the inside, see how the six sided shaft fits into a six sided plastic spool head? Now look in the far right side corner - that's what TroyBilt said it's supposed to look like. Any suggestions for adding the aftermarket head?