1) Create a local cylindrical coordinate system (easiest way to do this is about the working plane), the origin lying along the plane in which you'd like to rotate. Alternatively, if your geometry is applicable, just change to csys,1.
2) Go to Main Menu>Preprocessor>Modeling>Move / Modify>Volumes... and pick the volume.
3) In the dialogue box input a value in the Z-direction in degrees. Remember that the "Z" direction will depend on the initial orientation of the working plane - rotate the working plane to change this.
4) Switch back to csys,0
5) job done
Cheers,
-- drej --