How to delete / cut of a part of the ceiling

You can refer to this tutorial: https://www.homestyler.com/forum/view/1681508383154233345

Try to go to 2nd floor and select "Customise floor". The floor of 2nd level = ceiling of 1st level. m

You can't remove part of the ceiling. In plane view (2D), click on the room, right side menu-scroll to you see 'show ceiling' move slider and the ceiling for that room will be removed. Then use custom modeling to create ceiling where you need it.