It's an easy enough fix you know, just ask Mabel to make a pro design and then make a clothes item of whatever you messed up (in this case a shirt with long sleeves) choose to store the new design over the old one, then when selecting which part of the shirt you're working on you can drag the front part of the shirt over to the back with the stylus and pick "swap." It will cost the typical 500 bells of making a new design but other than that it shouldn't be a problem as long as you use the same character who you made the design with initially (you can't alter a design from a different character, even if it's your own alternate). I've made this mistake myself a few times.