You can simply poke a hole in the rusty one and pry it out.
Then get a new replacement and tap it in.
If it is in a bad spot where you can't get to it easily, they have expandable ones that are easier to install in tight spaces.
Use something with a broad tip--a screwdriver is apt to punch through the plug--to hammer on the edge of the plug, rotating it into the hole. Once it's rotated in, grab the edge with pliers and pull it out. The only thing to worry about with using a socket as an installer is that the plug will close up slightly when installed. If the socket is too large, it'll be pinched inside the plug.
If the engine's out and you can get to all of them, you should replace them all.