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You have to pry it open, but really careful. This is normally done with a parts separator. But I take it you don’t have one.
You can use your knife and gently stick it between the parts. Don’t use a twisting motion, as it will damage the edges. If you hold the part horizontal the move the knife in an upward and downward movement.
This way you’ll do as little as damage as possible to the part. Once there is a little gap, don’t go any further on that side, as it will put stress to the pegs in an angle which will surely break them.
Just move to the opposite side and start the procedure again. If there are more edges, go the the edge farthest away from the second edge, then the opposite edge from that one and so forth.
Of course, if you can find something else as a knife, but with an edge that can get between the parts, use that. Or if the knife has created a gap big enough for a tool with less chance of doing damage to get in the gap, use that tool from thereon out.
I hope this helps you out. Good luck and happy building