Freeze plug

I will assume the coolant is drained. Take a pry bar or screw driver and pry the old one out. Next, clean the opening of rust and dirt. Now you will have two options. The steel plug needs to be tapped into the engine block. If you have room, take a 2x4 and lay it flat against the freeze plug and tap it into place. Make sure it is tight and fully installed.

If you have limited access, they make a rubber plug that has a large washer on the outside and one built into the rubber. It has a nut that when tightened, squeezes the plug causing it to expand. These are much easier to install especially when there is limited access.

Once done, refill the coolant, start the engine and allow the system to pressurize. Check for leaks.