Iirc you need to find the closed doors in the second floor. One is in the room / level with the stairs, one is the the room / level right of it.
Then a key spawns in the room over the prison, with it you can open the closed room in the room / level with the stairs. Said room as a key and a numeric keypad (3429). With that key you can go into the other closed room and find another key. Then iirc you can open the bathroom in the first floor in the room / level in the very right. Due to the keypad stairs should have appeared near the bathtube. Down there you'll find the key for the backdoor; it's located to the very left.
If your health runs low you can replenish it by eating. Second floor, room / level to the very right. Save before eating, as some food have sleeping pills in it.