How do you adjust the timing belt on a Plymouth Reliant car?

A timing belt implies you are working on the 2.2 or 2.5 liter engine. The 2.6 liter engine has a timing chain.

If you are working on the timing belt, I would recommend getting a repair manual for your car. If you are attempting to re-tension an old belt, you should consider installing a new belt, but be prepared to unbolt one of the engine mounts. Having said that, there is a tensioner on the firewall side of the engine, which is hidden under a plastic cover. Depending on how many accessories you have, this cover is probably one of the more difficult items to remove in one piece from your engine. Once exposed, have a friend help you loosen and re-tension the belt (approximately 5/16-inch deflection measured half-way between the camshaft sprocket and the tensioner pulley).