Use a heat gun on low setting to warm up the plastic all the way around the seam. It doesn't take much.

Squeeze the sides of the case and you should see it start to open up slightly in the front. Use a guitar pick or plastic pick to wedge into the seam. A thin pick works best.

Once you get the first pick in, work it around to one side, then insert a second pick the same way as the first to the opposite side.

You may need to re-heat the case a couple times during this process. Keep squeezing the case and work the pics to each side. Doing so should release the clips and allow you to separate the two halves.

Be patient and don't use too much heat.