If you are working from a Windows PC, you should just be able to do this:
1. Open Explorer
2. Go to the folder containing the photos
3. View the folder contents as icons – large or extra large woks best, but medium also works
4. Right click on a photo that needs to be rotated and you should see a pop-up menu with the options “Rotate clockwise” and “Rotate counterclockwise”. Click as appropriate.

That should work and should apply to all programs.

On an iPad:
1. Open photos and view the photo
2. Touch “Edit” – top right
3. Touch the rotate / crop icon – middle right
4. Touch the rotate left icon – top right – once or three times
5. Touch “Save”

Sorry, don’t know how to do the equivalent in any other platforms.