I also can't find any scheduled interval for PS fluid replacement. I replaced mine because it looked kinda dirty when we inherited the car ('98 Accord 4-cyl).
Lift & support the front of the car so you can turn the steering back & forth.
There's a section of hose connecting to the cooler tube infront of the radiator. Look down low, under the PS reservoir. Unhook that hose & catch the fluid in a bucket.
Start the engine & turn the steering back & forth all the way a couple times to pump fluid out from the lines. This will make a real mess unless you're creative with some hoses to catch the fluid. Don't run the engine any longer than you really have to, maybe 30 seconds.
There's a fine screen down inside the reservoir, check that out. If you need, remove & clean out the reservoir. I've used stuff like SimpleGreen & hot water. Make sure you dry it very thoroughly. Don't forget to hook up the hose where you drained the fluid.
Hook the reservoir back up & fill it. Run the engine just long enough to turn the steering back & forth a couple times. The fluid will be foamy now... Let it settle (1/2 hour?) Add what you need & repeat this step a couple times. Eventualy the foam will settle out.