The Straighten layer can be used to fix a tilted horizon. Add a Straighten layer by clicking the Straighten icon on the Edit tab of the toolbar.
Click and drag on the image to start rotating it. While the image is being rotated, an alignment grid will appear. Use the grid lines as a guide for making features in your image vertical or horizontal.
Hold the Shift key while dragging to more precise control in order to make fine-tune adjustments.
If an exact rotation value is desired, use the Rotation decimal entry control to enter a positive or negative angle.

Note: The straighten tool can only rotate images by 45 degrees clockwise or counter-clockwise. To rotate the image further, use the Rotate layer.