Enter the mirror to rotate the world, or turn it upside down.
Grab the Mirror (or any object) with left click / Y (and throw it with right click / right shoulder button).
Rotate the mirror with the arrow keys or the shoulder buttons to align how you want to rotate the world.
Recall the mirror with F or X (on gamepad).
(There is currently no way to exit a level, you will have to restart the game. We apologize).