Upon completing the castle's quest you will recieve a key to unlock the upstairs and bedroom. If you already completed the quest earlier and load the game, the key should be added to your inventory :) If for some reason none of those work (which they should as I've tested both ways thoroughly) you can always pickpocket the castle steward, Avik, as he now also has a copy of the key to unlock the upstairs and bedroom areas :)

I thind the key and the quest