Use one hand to press together the short edges of the case (in the direction of the green arrows). This will create moderate gaps in the top rim on all four sides

With the other hand, insert a spudger or pick in each of the four gaps, and carefully lever up the inner plastic piece a little bit.

The clips in each of the four corners (location indicated by the red circles) aren't very tight and should be easy to release.

I used a metal spudger, but you should prefer a plastic pick as there's less chance of causing damage.