If you turn your camera 90 degrees to take a photo, Photos usually rotates the photo during import so that the image in the photo appears right side up. You can also manually rotate a photo.
Do any of the following:
Control-click the photo and choose Rotate Clockwise.
Double-click the photo, click Edit in the toolbar, then click the Rotate button 
.
You can rotate the photo multiple times to orient it the way you want.
Tip: To reverse the direction of rotation, Option-click the Rotate button .
You can also rotate a photo when using the crop and straighten tools. For more information, see Crop and straighten photos.