To edit/update the details (Title, Story, Tags, Category, Location, Adult Content etc) of a photo you have uploaded head to your Profile.
From there, click of the photo you want to edit to bring up in the large popup view, then click the ‘More’ button (3 dots) and select ‘Edit photo’ from the dropdown.
This will allow you to edit the details of your photo, but it will not allow you to replace the actual photo file. You cannot replace the photo file of a photo.