1. Literally, to climb steps to reach a higher level of a building. A: "Where's Susie?" B: "Oh, she went upstairs to bed." When you go upstairs, can you take the laundry basket with you?
2. To go to someone higher in a hierarchy, like one's boss, to seek their authority for a decision or other purpose. I'd go upstairs with that request—we don't have the authority to sign off on something like that.
See also: upstairs