The floor above; upstairs (the floor above the current one).
上階から水漏れがあって、天井にシミができた。
There was a water leak from the floor above and a stain appeared on the ceiling.
会議は上階の会議室で行われます。
The meeting will be held in the meeting room upstairs.
上階の住人に荷物を届けた。
I delivered a package to the upstairs resident.