Windows 10:

Open the photo on your PC. In the top right corner there is a set of tools. Click the arrow circling a dot to rotate 90 degrees clockwise. Repeat if necessary.

Windows 7:

Open the photo on your PC. At the bottom of the window there is a set of tools. The two circular arrows rotate the image 90 degrees clockwise or counterclockwise respectively. Repeat if necessary.

Mac OS X:

Launch the Photos app on your Mac. Find the photo and open it. Go to Image > Rotate Clockwise (or Counterclockwise) from the menu to turn it 90 degrees. Repeat if necessary.