Turkey baster is an item I keep in my professional tool box. sean is spot on if you want to replace the fluid in the reserv(ior).
However, if you want to replace ALL the fluid in the P.S. SYSTEM, you would need to do a FLUSH.
Locate the return hose and disconnect it at the back of the reservoir, let it drain off into a drain pan. You might have to extend the length somehow.
Raise the front wheels off the ground.
Take the cap off the reservoir and get PLENTY of P.S. fluid, maybe 3-4 quarts.
Start the car and keep the reservoir about half full while someone turns the steering wheel back and forth lock to lock, don't hold at the lock positions.
Repeat until clear fluid only is draining into the drain pan.
Reconnect return hose, clean your mess and you're done. HTH.