Someone else might need to explain this in more detail but the pedal is lowered by adjusting the length of the rod that attaches the pedal to the rear master cylinder.
Pull the split pin from the rod, disconnect pedal from rod, turn rod clockwise into master cylinder. Refit everything and check height of pedal. Repeat as necessary.
However, be aware that if you have the pedal too low, it will hit the exhaust before the rear brake starts to bite/work.