All along the edge of the assembly, there are plastic clips that are molded into the casing. Although these are quite tight, you can pry these clips carefully but firmly, one at a time, so that the case begins to open along this seam.
When I first started doing these I would use small nails to try to prop each clip to stay open as I moved from one to the next; however I found that it was ultimately not necessary as I was able to keep the previous clips open on their own.
There is one area along the case which has some additional plastic molding that gives some trouble releasing. Eventually with a bit of extra effort that area will also open up.