Here are some methods:
1. Dip a cotton swab or edge of a cloth into some rubbing alcohol. Rub it on the back of the shirt right behind the iron-on letter. Wait about 10 seconds and then try to peel of the letter. Repeat if necessary.

2. Use a lint brush or a wire brush to scrub the last pieces off of the shirt. Brush smooth and gently until the pieces come off. The pieces should start to come off in little balls.

3. Wash the shirt in hot water and scrub with hands or wire brush. any remaining glue should come off of the shirt.

4.Use the iron method if none of the other methods work. Wet the shirt and put it face up with a piece of parchment on top. Put your iron on the cotton setting and rub over the parchment. Peel off the parchment. It should have the transfer on it.

5.If there are still little bits of the letter on there, then repeat step four or use steps 1-3.

Hope I helped!!