Should be no different than using stairs anywhere else. After the surgery, you will go through physical therapy and you'll have to learn how to walk all over again. You'll recover quickly, but they'll have you walking up and down steps. In a short while, you'll be ready for the Boston Marathon.
Don't sweat the steps. You'll do fine!
If you are having only 1 knee replaced, it's the "other" knee that will be screaming ... "NO! NO! NO!"... :B