Hi @Voli0
You need to go to the listings tab and click the photo tour. There you delete all the photos of the "old" bedroom and delete the bedroom folder. Put all your new photos, including photos of the bed, in the "living room" folder. In the living room folder, below the photos, you can select the correct size of bed under "sleeping arrangements".
Your listing should then change from one bedroom to studio. To check whether it displays correctly, click the listing in the listings tab, and click "View" to see what the front page of the listing looks like after the changes.