Hold the door open with a doorstop and have an assistant ensure the doorstop stays in position. Stand on a stepladder beneath the door closer and locate the pair of screws on the left end of the opener.
Turn the upper screw clockwise to decrease the amount of force applied by the closer when the door is released from an open position; turn it counterclockwise to increase the force. Repeat the process with the lower screw, which regulates the latching force.
Step down from the ladder, remove the doorstop and allow the door to swing closed. Notice how the door closer behaves differently with its new settings and make additional adjustments, if necessary, until it swings closed at a controlled speed and gently latches.