You can try to fix it but only if you feel comfortable doing so. The pressure relief valve is just a spring that pushes on a diaphragm. When the pressure inside the chamber produces a force greater than the force the spring the diaphragm opens releasing the pressure until pressure inside the chamber lowers.
Assuming the spring is still good, you just need to tighten the adjustment on it so that the spring is more compressed. Almost all relief valves are adjustable, and although they come in many different shapes and sizes most all work on the same design principle. If the spring is bad (which is rarely the case) then you just need to purchase a new relief valve of the proper pressure gate.
Note: The relief valve is a safety feature and tightening it too much so that it would never open can compromise the safety of your setup if the pressure for some reason ever became too great.
I'm reading this correctly right? You said the left gauge CAN go over 5 just fine? Its when you adjust the right gauge you have the problem, correct?