Click on the room where the ceiling height should be adjusted. You’ll see options of changing the height and thickness at the bottom of the screen. Simply enter your desired dimensions and let Planner 5D do the rest for you.
Don’t forget that these dimensions are only applied to a chosen room. If you need to have a higher ceiling in every room of your apartment, you will need to pick each room and adjust the ceiling height there one by one.
Here's a short video that might also come in handy: