water change and/ or PH Buffer. Dont know how low it is and how safe it is to raise it fast but do a water change first, if still a little low then add some PH buffer. Try to find out what the reason is also that it went low in the first place as buffer is really temp band-aid
If you have glass tops, remove them and replace it with "egg crate." Increase your water flow, to make sure that the water moves across the surface of the aquarium.
Buffers do not fix pH. The easiest way to increase pH is to do at least a 50% water change.