1.
If you go upstairs in a building, you go up a staircase towards a higher floor.
He went upstairs and changed into fresh clothes...
? downstairs
ADV: ADV after v
2.
If something or someone is upstairs in a building, they are on a floor that is higher than the ground floor.
The restaurant is upstairs and consists of a large, open room...
The boys are curled asleep in the small bedroom upstairs.
? downstairs
ADV: be ADV, n ADV
3.
An upstairs room or object is situated on a floor of a building that is higher than the ground floor.
Marsani moved into the upstairs apartment.
...an upstairs balcony.
ADJ: ADJ n
4.
The upstairs of a building is the floor or floors that are higher than the ground floor.
Frances invited them to occupy the upstairs of her home.
? downstairs
N-SING: the N