help me please

hello

Help me please...
I have two "postgres" user. How to delete the first one ???

*******************
# select * from pg_shadow where usesysid=1;
 usename  | usesysid | usecreatedb | usesuper | usecatupd
|               passwd                | valuntil | useconfig
----------+----------+-------------+----------+-----------+-------------------------------------+----------+-----------
 postgres |        1 | t           | t        | t
|                                     |          |
 postgres |        1 | t           | t        | t         |
md53532747417351142d5270721fcf740ed5 | infinity |
(2 rows)
*******************

*******************
# select * from pg_user where usesysid=1;
 usename  | usesysid | usecreatedb | usesuper | usecatupd |  passwd  |
valuntil | useconfig
----------+----------+-------------+----------+-----------+----------+----------+-----------
 postgres |        1 | t           | t        | t         | ********
|          |
 postgres |        1 | t           | t        | t         | ******** |
infinity |
(2 rows)
*******************


I tried
    delete from pg_shadow where passwd is null;

but it's not working.

Help me please
oleg