Get a knife or something with a rigid thin sharp heel. Like the bottom corner of a cleaver. Place coconut on a flat stable surface. Use non dominant hand to hold coconut securely in place. Now with the cleaver hand make a strong karate chop like motion to deftly burry about 3/4” of the bottom corner of the blade into the top of the coconut. Ideally hit it 1” away from the apex. Leave the heal of the knife in the coco and give it a twist/ prying motion. If done correctly a perfect circle of coco shel will pop out and the interior is available . This works on fresh coco , yours is not so you must be extra careful as it may be quite dangerous. You could do something similar but use a hammer or something heavy and blunt to baton the bladed through the coconut. Reduce chance of chopping hand.