Hi Elvin, could you post your 3DM model file with the cylinder you want to rotate in it? That would help to see what particular situation you've got. But most likely as shayne writes above you'd select your cylinder and use the Transform > Rotate command for that. Probably you'd want to pick the rotation origin point in the Front view with object snaps enabled and type -45 and push Enter to do the 45 degree angle rotation in the direction you want.

- Michael