On a clean, hard surface, lay the rear panel on its side. Carefully but firmly push down on it, rolling the entire lip side back into its proper spot.

It may be necessary to do this multiple times in order to achieve optimal straightness on the sides. It is better to have the edges of the case pushed in slightly too far rather than not far enough, because the reseating of the front panel will bend the rear panel into its correct alignment.

Now that the rear panel is back to a beautiful condition, you can move on to repairing the iPod!