How do you open a mature coconut?
Drain it first! There are three round indentations on the top of the fruit -- one of these holes will be softer than the others. With a paring knife, chopstick, or screwdriver, poke a hole into that one, then drain the water out or drink it with a straw.
Once the water is drained out, use the blunt end of a big knife to whack along the equator. When you’ve gone all the way around, the coconut will split open in half.