You can change it with the car running. I do it on the Supra. Just take the return hose going into the reservoir, put it into your discard container, start it up, and pour in PS fluid until the stuff coming out of the return is clean. Using clear tubing w/ that job helps a lot.
If you don't, the stuff starts to get burnt and you'll pick up some debris in there anyway. Eventually it clogs up the screen at the bottom of your PS reservoir and impedes the flow. If you hear a whining sound when you fully turn the wheels, esp when cold, you know you're either low or not getting enough flow. If it gets that far you usually have to take out the PS res to clean it.
Better to change it now.
EDIT: don't forget to bleed it when you're done, especially if it gets a little low while you're changing it. If you can, park on a slight incline w/ the PS res cap off and turn the wheels side to side until the fluid in the res isn't getting foamy anymore. Make sure to keep topping it as you do this, since you want to replace that air w/ fluid. This part's good for a 2 man job.