Whether you need to replace or refill your power steering reservoir depends on your vehicle, the fluid that you are using, and importantly, what you find when you open the reservoir cap.
If you find that your fluid has changed color, has a burnt smell, or contains debris or particles, then the fluid needs to be changed. However, these are also signs that there may be further issues with the power steering. You will need to check for leaks, corrosion, and damaged seals before replacing the fluid.
If there is nothing wrong with the fluid and no signs of corrosion or leaks, then you should be clear to top up the fluid levels. However, you should ensure that you are using the correct fluid and that it is the same as the fluid that is already in the reservoir. Mixing different types of fluids can cause problems. If in doubt check with your car's manual and check the manufacturer's details on the fluid itself.