Rotate the cube
Select the Control > Rotate tool. Click and drag as shown to rotate the cube. Notice how it rotates on all axes simultaneously and how difficult this is to control.

Undo the last operation
Use the Edit > Undo (Command-Z) menu item to reset the model back to the way it was before rotating.

Constrained rotation
Using the rotate tool was cumbersome and difficult to control. That's because we were trying to rotate in three dimensional space using a two dimensional pointing device (mouse). To make things more manageable, we can constrain movement to one or two axes. Click on the X Axis and Z Axis buttons at the bottom of the screen to disable them.