The grey lever near the inlet/outlet fitting adjusts the flow. Pull it toward you to slow the flow. Push it all the way in for full flow (the flow meter to the left has a white ball in it that drops as the flow slows).
To disconnect the tubes for servicing, use your right thumb to push in the tiny red button to the right of the grey lever and pull the lever all the way toward you until the inlet/outlet fitting pops off (the fitting will be closed, and the water will stay in the tubes).