Upstairs is a natural sign. Take your dominant hand and form it into an inverted 'V', with your index and middle fingers pointing down like two legs. Then motion them upward diagonally as each "leg" takes turns walking up an imaginary staircase.
Teach your baby the sign for upstairs when indicating that you're about to go up to a different floor of your house or any other building. Teach it in contrast with downstairs.
Use the upstairs flash card contextually when your baby has the opportunity to climb up and down a set of stairs. Perfect for babies who live in a two-story house or building.