Is your photo professionally taken? If it's correctly taken then there should be nothing to adjust. If it's not, there maybe dark or bright spots and no postprocessing will do it right. The lighting must be uniform, no shadows.
If the photo is professionally taken, go back to the store and tell them they need to retake it.
If not professionally taken, get it professionally taken to avoid issues including the risk that it will be later refused even if accepted by the site.