Unfortunately, there is no easy fix. Removal and replacement is the best. Second best would be one of those concrete "jacking" companies that drill a hole in the existing and pump new concrete underneath, lifting the settled area. They may not be willing to do so on a pool because of the possible lateral forces that may develop on a pool wall.
I don't believe there is any long-term fix by going over the top of the existing. Others may be aware of something I'm not.