Place a stepladder beneath the opener's motor head. The force adjustment dials are located on the right side of the unit; icons beneath the dials indicate which one corresponds to upward and downward force.
Turn the dial marked with a downward arrow icon containing the letters "KG" clockwise with a screwdriver to increase pressure or anticlockwise to reduce. The pointed shape of the dial makes it easier to see how you are adjusting the pressure.
Test the door with increased or reduced force. You should increase the amount of force if the door reverses during closing or reduce it if the door moves with too much force. Move by increments of one until you find the right setting.