Shut off your mower and make sure the blade has completely stopped turning.
Turn the mower upside down so that the bottom of the deck is facing up toward the sky.
Press the lever next to each wheel and move it up or down, depending on if you want to cut your lawn shorter or longer. Use your screwdriver to loosen the bolt that secures the height of your mower wheels. Move it down to cut it shorter and move it up to cut it longer.
Check each wheel to make sure that they are each at the same height. Turn over the mower and start it again.