Click on the room where you want to adjust the ceiling height. You'll see options for changing the height and thickness at the bottom of the screen. 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.
Note: the next floor will be built on the highest wall of the previous floor. That means if you need to have different wall heights on the first floor, make sure it doesn't conflict with the second floor. If it does, you will see the second floor "floating" above the first floor at some distance. That is the sign that some of the first floor walls are higher than the others.