You can't reuse normal iron-on patches, but they do make special "reusable" ones that serve just that purpose.
I don't know if the patch is reusable. It depends on the typeof patch you have. If it can be reused you can buy double sided tape to reapply the patch. This tape is also iron on and works GREAT.
As for the removal of the patch you can try the method described below.
1). Set your iron for the highest heat setting that the garment can tolerate.
2). Iron over the patch, watching the garment carefully for signs of scorching. Continue ironing for
about 30 to 60 seconds, at which time the glue on the back of the patch should loosen.
3). Grasp a loosened edge of the patch between your thumb and fingers. Slowly begin to peel the
patch from the garment. You'll probably need to reheat the patch a time or two to get it off
entirely. Don't iron directly over any exposed glue residue that may remain on the garment.
4). Carefully apply some alcohol to any remaining traces of the patch's adhesive. Don't iron over it.
5). Launder the garment in cold water.