For a non-adjustable bench, you can get cuts of 1x4 wood, or similar and stack them under the legs to make the bench taller. You could also use parts of a phonebook.
Many adjustable benches will have a knob at one or both sides of the bench, you turn the knob one way to raise and the opposite to lower. If it is difficult to tell which turn direction is which, sit on the bench and turn the knob. One direction will have more resistance than the other, the direction with the greater resistance raises the bench.
I've also come across adjustable benches that have been "fixed" into a certain height. Some of these adjustable benches have a tendency to wobble, and locking them to a certain height may be a solution to the wobble. Look underneath if something has been wedged in the mechanism.