- clicking on the photo's "Edit" icon
- clicking the "Crop & Rotate" icon
- clicking the "Rotate" icon once for every 90 degrees clockwise
- click "Done"
- click "Done" (again)
That's an incredibly tedious process, especially if several images need to be rotated.
There is a much simpler procedure. Click on an image in Google Photos, and press [shift]+[R] on the keyboard. The image will rotate 90 degrees counter-clockwise. Repeat the shortcut to rotate another 90 degrees as necessary. The change is committed with no further actions.
Pressing the [left arrow] and [right arrow] keys on you keyboard, you can advance to the previous and next images, and repeat the rotate operation. Thus, you can quickly rotate a number of images.