Upload your photo, and under Basic Edits choose "Rotate":
Click on the "Straighten" button...notice how it places that grid over the photo?
Now, drag the straighten button to the left or the right, and it will rotate your photo either left or right. You can use the grid to guide you as to how straight your photo is within the frame:
PS - I'm not super picky about this all the time...sometimes I do take crooked pictures on purpose! ;)