Use the Tools Button and click Open in Editor, then Rotate.
You'll see the following:
Choose the direction you would like to rotate your selected photos: Left, Horizontal Mirror, Vertical Mirror, or Right. Click Save once you have everything rotated to your liking!
To rotate another photo, use the left and right arrows to move through your images. Or return to your gallery and select the next photo you’d like to rotate.
Rotation will occur in the background and may take several minutes. In the meantime, you can continue to use SmugMug. As you browse your gallery or click refresh while viewing it, your newly rotated photos will gradually replace those that are currently there.