just add a length of hose to the return line and route it into a drain pan or jug. While car is running -- pour new fluid into tank while old fluid runs out. Need as least a gallon for this job.
Best done with a friend to try and avoid getting air into the system. If needed, system can be bled by putting the car up on jack stands (ps system all sealed up) and slowly turning the steering wheel back and forth with engine running.....