Well at least we know some of the information. There are freeze plugs in the block, to include the rear behind the transmission, and in the head.
It depends on which plug is bad as to how to replace it.
usually it involves digging the bad one out and knocking a new one in. There are tools that are made for this, and some plugs can be in bad places. I have replaced many with a socket that fits, an extension, and a hammer. It has been necessary to use a warble extension on more than one occasion. It can be a tedious job.
Also, the general reason that a freeze plug goes bad is rust. This is caused from using water and not coolant. Coolant has an anti-rust inhibitor. This could mean there could be more than one with a problem.