If you have access, they are easy. Just take a punch or straight screwdriver, poke a hole in the plug and pru it out. Make sure to drain the coolant or you will get wet. Clean the opening and take a new one, and tap it into place. I always use a small 2x4 to keep it straight and flush so it doesn't go in on an angle. Then I tap on the wood.
Let me know if you have other questions.
NOTE: If you would rather, they make a rubber plug replacement. You slid it in and tighten a nut on it until it tightens in the hole.