Freeze plugs are installed in the block of your engine in part to prevent damage from coolant freezing in the block. If you properly maintain your coolant levels and proportions they should never have to serve that purpose. But, they are also necessary as part of the casting process of the engine block so it is not possible to build an engine without freeze plugs.
Occasionally a freeze plug will fail, most likely due to rust from the inside of the block. In such a case it will need to be replaced. If you can get to the freeze plug with enough room to swing a mallet to drive the new plug into place it is not a difficult thing to do, however it is most likely that you will have to remove the engine or the transmission, or both, from your vehicle in order to do this.
Freeze plugs are located all the way around the sides and the back of the block. Sometimes freeze plugs are also located on the front of the block, in which case they will usually be behind the water pump or otherwise hidden by accessories. While getting access to the freeze plug is beyond the scope of the article here you need to keep in mind that this part of the job should only be attempted by an experienced mechanic. If you have never removed an engine from a vehicle this might be a job best left to professionals.
When you have access to the freeze plug you need to remove it. Using a punch to one side of the plug strike the punch sharply with a hammer to tilt the plug in the hole. Grab the side of the plug that is sticking out and pull it loose with a hammer.
Before installing the new freeze plug you will need to make sure the edges of the hole are clean. Using a piece of emery cloth, lightly polish the edges of the hole removing any rust or other contaminants.
Install the new freeze plug using a seal driver or a socket that fits snugly into the cup of the plug. Strike the driver or socket sharply with a hammer to drive the plug into place. A proper sealant is recommended around the edges of the plug to help prevent leaks.
Since the labor of removing the engine to get to the freeze plugs is the majority of the cost or work involved in this repair it would probably be a good idea to replace all of the plugs at once. The other plugs might not be leaking now but they almost certainly are just as old and nearly as damaged as the plug that was leaking.
As an alternative for emergency repairs rubber plugs are available. Fairly reliable for an extended period of time, if you can get the old plug out this might allow you to do the repair without removing the engine. Simply remove the old plug, install the new rubber plug and tighten it with a wrench. This obviously won't help for freeze plugs that are hidden behind the transmission or otherwise difficult to work with but it could work in your specific situation.