You can curve a ceiling edge using the Curve tool. Once the tool is active, you can click and drag the edge to curve it, or select a point to curve to.
To curve a ceiling edge by clicking and dragging:
Click on the edge you want to curve.
Right-click and select Curve, or select Edit > Modify Elements > Curve.
Click and drag the ceiling edge to the desired curve.
Release your mouse button.
To curve a ceiling edge to a selected point:
Click on the ceiling edge you want to curve.
Right-click in and select Curve, or select Edit > Modify Elements > Curve.
Select the point you want to curve to. The ceiling edge automatically curves to the point.
Click to finish.