SQL> select * from test;

        ID ENAME
---------- --------------------
           david
           john
           martin
           rocky

SQL> update test t set
  2    t.id = (select x.rb
  3            from (select t1.ename, row_number() over (order by t1.ename desc) rb
  4                  from test t1
  5                 ) x
  6            where x.ename = t.ename
  7           );

4 rows updated.

SQL> select * from test order by ename desc;

        ID ENAME
---------- --------------------
         1 rocky
         2 martin
         3 john
         4 david

SQL>