replace.sql

mysql> UPDATE files SET href = 
          IF(POSITION("assignments/uploaded_files" IN href) ,
             CONCAT(
                "http://www.assignmentexpert.com/uploads/users_files" ,
                SUBSTR(
                   href ,
                   POSITION("assignments/uploaded_files" IN href) + 26 ,
                   LENGTH(href) - 26
                )
             ) ,
             href
          )
       LIMIT 5000;