Core Plugs

It would probably help if you mention the engine involved.

In general, 'freeze' plugs, or Welch plugs are extremely simple to remove, if you can get to them. A sharp punch or a screwdriver can be used to puncture them, and you just leverage them out.

Tapping in the new set is easy if you have a socket wrench or tool of proper size to put inside th concave part of the plug, and gently hammer them in.

If you can get them out, but dont have room to tap in the new ones, you can either lift the engine, disconnect motor mounts, etc, OR you can install expandable rubber plugs.