When in the Edit area of your online design:
You should see your shirt on the left and the design options on the right.
Notice the Back box below the shirt.
Simply select the portion of the design you want to move to the back and drag and drop to the Back box.
Notice how the Back box turns green
By click on the box and you can toggle back and forth from the front of the shirt to back and edit each in kind.
Notice that the Back box now reads Front and displays what is left on the front of the shirt
If you want the whole design moved, just select the rest and drag and drop into the back of the shirt as described above.