Piston rings should be changed by a knowledgeable mechanic. The engine must be fully disassembled to change the rings. You also must change the bearings on the crankshaft and the connecting rods. Replacing the piston rings is considered a engine rebuild and you cannot just change them as you must ensure that the cylinder is true, meaning that it is straight from the top of the cylinder to the bottom, and that the cylinder is perfectly round, as well. This requires a special set of micrometers, so it's best to take the block to a machine shop to have it cleaned and checked. Replacing the rings is relatively quick, if someone is dedicated to the job. It can take several weeks, if a mechanic is doing the job for you, because it is a very intense job, requiring engine removal, disassembly, and as previously stated, possible machine-work.