please help on how to rotate the mesh in blueprint
What exactly do you mean by Rotate?
Would you like to rotate it at a constant rate? Use a RotatingMovement component.
Do you want to rotate the mesh inside the Actor editor? Since the Mesh is the Root component of that Actor its rotation is bound to WorldSpace, you need to have a separate component to act as the Root such that you can add the Mesh as a child and then you will be able to rotate it in LocalSpace. Ideally a simple Scene component can suffice for a Root component.
To rotate a component or actor, press the Space bar until you get the Rotation gizmo appear.