Make sure there's an upstairs and downstairs litterbox, so she can easily get to one without navigating stairs. She will probably benefit from a litterbox with shorter sides, so it's easier to get in and out of. Better to do this before she starts "forgetting" to use the litter box because it's too much effort.
She might like it if you carry her downstairs in the morning, and back upstairs at night so she can be near you without having to climb stairs.
Crouching to eat may not be as comfortable as it used to be, so you might want to try a raised food dish like this.
If she likes to sleep in a cat basket or bed, check to see if the sides are too high for her.
Also, look around at at her favourite places -- are they easy for her to access? Jumping up to that warm sunny windowsill may be more difficult now. But if you put a chair, footstool, or something like that next to the window, she can jump onto that, and then onto the windowsill.
If you have a boisterous dog or other animal that might make her nervous, it could be more difficult for her to escape them by jumping up out of their reach. You may want to make sure there are safe spots at ground level around the house. This could be as simple as a box with a cat-sized hole cut into the side for entry.