Relief valve leaks sometimes causing small flood, unit is rheem 50 gal gas fired unit. Thermostat is set at the mid setting, water tempurature is not exessive. Should I change the relief valve

If the pressure is below the valve rate and it is leaking then yes change the valve. If the pressure is over the valve rating then find the fault and make the repair.

There is a good chance the pressure relief valve is just doing its job. If you have anything that keeps water from flowing out the inlet, such as a back flow preventer, and don't have an expansion tank, the excess water from expansion has to go somewhere. Does the flood come when the tank is recovering from a hot water draw? If you take a shower, is the water coming out about the time you get dressed again and make it to the heater? Does the water quit coming out about the time the burner cuts off? A new valve may be a waste of money.