how to change photo from portrait to landscape

To rotate a photo from portrait to landscape in Photos you can simply select the thumbnail and use the keyboard shortcuts ⌘R or ⇧⌘R to rotate clockwise or counter clockwise. Before you do it, I would duplicate the photo with ⌘D and rotate the duplicate. Then use the copy to order your prints.

You could edit the picture in the Photos app (by cropping it or rotating it) then upload the modified picture. If you wanted keep the original in your library, either duplicate it before editing it or revert the edits after you've uploaded it.