Water heaters do not live forever. Start shopping.
While it is possible that you have a leaky fitting near the water heater, (which could be fixed) the most common cause for a leaky water heater is that the water heater itself has started to fail, and the only option there is replacement, as the corroded tank is not a replaceable part.
Depending where you are, there are some pretty good deals on more-efficient water heaters available in some areas.
Most temperature and pressure relief valves are not adjustable, and in any case the pressure inside should just be the water pressure of your house, so if you could lower the relief pressure, it would leak all the time.