Locate the adjuster on one wheel. It's between the wheel and the body of the lawnmower, and it has notches into which a raised lever is positioned.
Move the lever from the notch it's in by pulling the lever toward the wheel. Slide the lever forward for a higher cut, or backward for a lower cut. Secure the lever at the desired cutting height by pushing it into the appropriate notch.
Repeat this process for the other three wheels.
Double-check that all four wheels have been set at the same height after you've finished adjusting them.