Power steering fluid is just fancy ATF most of the time, so if it's brown, it probably oughta be changed.

I've not yet done this on either of my volvos, but on other cars, I simply popped loose the low pressure return hose from the reservoir, capped the nipple on the reservoir, fired up the engine and was prepared to dump in fresh fluid while somedboy SLOWLY turned the wheel. The return hose was obviously run into a bucket to capture all the old stuff.

I flushed with a quart or two of cheap stuff, then added Mobil 1 synthetic ATF (in accordance with mfg. spec for that car). No matter what fluid is used, power steering systems don't use a ton of it.

I'd flush.