To rotate a layer, make sure to select the layer with the Select tool (v) first.
When the layer is selected, it will show a dashed rectangle around it, along with 8 resize handles and 1 rotation handle above.
Click and hold the rotation handle. While holding, move the mouse around the layer to get to the desired angle. Release the button to complete rotation.
Hold the Ctrl key to rotate by 15 deg steps.
Hold the Shift key to rotate by 5 deg steps.
Hold the Alt key to rotate by 1 deg steps.