How to rotate 3d board?

I know that I can move it by left-click and hold and drag. I can resize it by turning the wheel (middle button) of my mouse. But how do I rotate it?

Hover over one of the corners of the board, and you'll see the cursor change. Then, left-click and hold to grab the corner, then you'll be able to rotate the board.

Or use the arrows on your keyboard.