I recently did this,
I don't know if this is the "right" way but I connected a piece of cleear tubing to the bleed screw on the control valve that attaches to the rear sway bar fired up the engine and opened the valve a little untile fluid started comming out. While this was bleeding I slowly added fresh fluid to the resevoir. My old fluid was so dark that it was easy to tell when I hit clean fluid. It takes a little running around to do it this was. You can do yourself a favor by pulling as much of the old out of the resevoir and adding fresh on top before you start so there is less to purge. There has to be a better way-be sure to never let the pump run dry! Good luck