not tested.. just a concept....
UPDATE table_name SET field_name=REPLACE(field_name, "find string", "replace string") WHERE id_field IN (SELECT id_field FROM table_name WHERE field_name LIKE '%find_string%')
OR
UPDATE table_name SET field_name=REPLACE(field_name, "find string", "replace string") WHERE field_name LIKE '%find_string%'
mySQL Docs:
6.3.2 String Functions