Water level in bowl is too low so it's a problem keeping it clean. How can I raise the water level?

All toilet bowls have a preset water level determined buy the design of the internal s-trap and can not be change. However if the water level in the tank is not as high as it should be this will affect the water put into the bowel as the bowel in refilled by the flush valve and if it is set to stop before it get filled to the proper setting the bowl water level may be low.
A quick test is to see if the water level in the tank is up to the line in the tank most all toilet tanks have a line either stenciled or marked with a indentation in the porcelain. Another way is to mark the level in the bowel with tape or a marker then pour water slowly into the bowel not fast enough to flush it wait to see if it stays higher then the mark you made or settles back to that level.