If a project design is modified, you may wish to change the DXF for a particular floor.
Load your project into Design Database, go to CAD Input and select the floor you wish to change. From the menus, select Defaults, then Default data.
From the default data dialog, examine the right hand side for the "..." button in the DXF File Name section.
This will allow you to select a new DXF file for your floor.
If loading a new DXF appears to delete your rooms, it is likely that the new DXF has a different origin to the old one. Your rooms will not have been deleted but moved out of the current view space. Check this by looking at your project in 3D - the rooms will still exist.
Ensure you have the same origins for your old and new DXF file. For more information on how to move your rooms so they align with a new DXF, please see this article.