An iron-on patch is applied with a heat set glue. To remove the patch, you need to apply heat to soften the glue. You can do this the same way you applied the patch -- with an iron. Turn your iron to the medium setting and move it slowly over the patch. Try to peel up the edges of the patch after 20 or 30 seconds of ironing. Keep ironing as necessary until the whole patch lifts away. This may take several minutes.
You can also remove the patch with a blow dryer or a fabric steamer. Apply the heat until the patch begins to peel away. When you get to a point on the patch that is still stuck to the fabric, apply the steamer or blow dryer again until the glue is loosened.
Use a Chemical Solvent
If you have a tough patch that won't come off with heat, you may be able to remove it with an adhesive solvent. Test the solvent on a small corner of the fabric before you attempt this step to make sure the solvent won't damage the garment. If there are no issues, turn the garment inside out and apply the solvent to the area at the back of the patch. Use a towel or a cotton ball to apply the solvent until the fabric is saturated. Let the solvent sit on the fabric for three to four minutes or as long as the instructions specify. Try to loosen the patch. Apply more solvent as necessary. Use the solvent in a well ventilated area.
Remove the Remaining Glue
After you remove the patch, you may notice residual glue on the fabric. You can remove this with the adhesive solvent designed for use with fabric. Again, test any solvent on a small area on the edge of the garment first and work in a well ventilated area. Use a towel or cotton ball to apply the solvent directly to the glue. Allow the solvent to sit for three to four minutes, and then use a clean towel to wipe it and the glue away. Reapply the solvent as needed.
Wash the Garment
Even if you have completely removed the patch and the glue, there may still be traces of the solvent or the glue, or dirt that was trapped under the patch. Wash the garment according to the care instructions to remove all traces of residue. You may also find that the garment is darker where the patch was located, and washing can help to blend the area. It may take many washes until the spot fades to the same color as the rest of the fabric.