DECLARE
   str   VARCHAR2 (1000);
BEGIN
   FOR cur_r IN (SELECT table_name, column_name
                   FROM user_tab_columns)
   LOOP
      BEGIN
         str :=
               'UPDATE '
            || cur_r.table_name
            || ' SET '
            || cur_r.column_name
            || ' = replace('
            || cur_r.column_name
            || ', ''C'', ''X'')';

         EXECUTE IMMEDIATE (str);
      EXCEPTION
         WHEN OTHERS
         THEN
            NULL;
      END;
   END LOOP;
END;
/