Select the item you wish to rotate.

Four icons will appear at the bottom and center of the canvas. The second one, two arrows going in a circle, is the rotate icon. Click this button.

At the bottom of the screen, a slider enables you to rotate your item. You can click and drag the dot to rotate the item, or you can click on any point of the line. Above the slider line to the left, you can see the degree of rotation.

Once you've finished rotating the item, click on any blank area of the canvas or press the esc button on your keyboard to deselect the item.