There should be a plastic screw at the top of the fill valve if you have the newer one.
It should turn by hand. Just adjusted mine a little the other day.
Got a pic of your tank? Notice how the water level is about an inch under the overflow tube. When I installed a new one in ours, I had to hacksaw the tube to make it shorter. The water would go into the tube far before getting up to the handle. That's like another inch above overflowing into the tube. Adjust the screw so that the water stops about an inch below the overflow tube. Is your overflow tube longer than it should be? Did someone recently install a new one or something? That's odd.