First you must select a session that is of type 'Proofing.' This will be any session that isn't an Edited Session.
Your photographer will allow you to select up to a specific number of images for editing as per your contract.
Click the 'Mark for Edit' button to indicate to the photographer that you want this photograph edited.
To expand a photo, click the image.
Once you have finished with your selections, click the 'Finalize Selections' button at the top and confirm your selections.
If you have already choosen your limit for editing and you have changed your mind you will have to email the photographer to make changes as they have possibly already started working on it.