Those would be "core plugs", often (mistakenly) called "freeze plugs". The stock plugs are steel and eventually rust away. They are cheap and easy to replace, except for one... Here's how:
1, Remove the plug by placing a flat blade screwdriver on the lip and start tapping with a hammer. What is supposed to happen is that the plug rotates and can be removed with a pair of pliers. What will actually happen is that the screwdriver will slip of the the lip and pierce the rusted out plug face. No big deal, just pry it out as best you can, being careful not to gouge the block/plug mating surface.
2, Find a socket that fits inside the new plug and use it as a driver, and tap the new plug in place with a hammer.
Some people smear a thin film of gasket goop around the sealing edge of the plug. I don't think it makes any difference if the mating surfaces are in good shape.
Now the bad news. If one is leaking, the others are very close to leaking and should all be replaced. Easy enough, except for the plug on the very back of the engine, behind the flywheel/flexplate. If the head is the original head, the core plugs on the head should be changed too. I've never tried to replace the back block core plug with the engine still in the car. It could be done in the car if you pulled the tranny and everything off the back of the engine. No fun. Most people that change core plugs in-car just skip this one and hope for the best. It's a big one, and often a little thicker than the rest.
Good luck.
Rick(wrench)
