Replacing power-steering fluid generally is not listed among the regular maintenance items that should be performed, so on most vehicles replacing it is at the owner's discretion.
When you take your car in for routine service you may hear an urgent pitch for having your power-steering pump flushed and filled with fresh fluid because the current fluid has turned dark. Bear in mind that engine oil and transmission fluid also become darker after a while, so a deeper shade of red doesn't mean the power-steering fluid is bad. Before you jump at paying for this service, see what your owner's manual or maintenance schedule says. You probably won't find mention of changing the fluid.
You should check the power-steering fluid reservoir monthly to make sure it has the proper amount and that the power-steering system isn't leaking. Reservoirs on many vehicles are the see-through plastic type, so you don't even have to remove a cap to check the level. Consult your owner's manual for the location of the reservoir in the engine compartment for help. You also should check the manual for the type of power-steering fluid that is required. The manufacturer may call for a specific type of fluid instead of a generic type found at parts stores.
Losing sleep worrying about the power-steering fluid in your vehicle because of its appearance or age? You could refresh it without draining all the fluid and flushing the system, but that may be more complicated than you're comfortable doing yourself (or paying for). Instead, find a turkey baster that will fit inside the reservoir and drain as much old fluid as you can. Then, you can refill it with fresh fluid to the proper level. You won't get all the old fluid, but do this a few times and you should be able to replace most of it.