If you are asking what you should say to someone who is on the elevator in order to find out which way the elevator is going, you can simply say "are you going up or down?" Upstairs and downstairs refers to floors above or below you, you could tell someone "I gotta go upstairs to get my umbrella" or "I gotta go downstairs to drop something off" or "I have to go upstairs and tell them to turn down that music". Upwards and downwards is a movement direction so it's used more when explaining how to operate something or when doing exercises.