Ahh branded PC casing is hard to open. Remember last time I
tried fixing my friend's branded PC, it took me about 15mins+ to
figure out how to take the side panel off. In the end, I was only
able to take one side of it off (this was easy, took me less than a
min), however the other side I just could figure out how to take it
off.
Anyway, I remember my friend's OC came with a manual that teaches
you how to open the casing (though the manual of my friend only
teaches me how to take one side of the panel off). I think you
should have it, go search around and see.