Configuration RMAN 10g to set Obsolete Backup

Default obsolete is 1 day, it will change to 2 day :

When got the error below :

SQL> alter database archivelog;
alter database archivelog
*
ERROR at line 1:
ORA-01126: database must be mounted in this instance and not open in any
instance

Try to do it :

SQL> shutdown immediate;
SQL> startup mount;
SQL> alter database archivelog;
SQL> alter database open;
SQL> archive log list;
Database log mode Archive Mode
Automatic archival Enabled
Archive destination /u02/archive/archive01
Oldest online log sequence 2
Next log sequence to archive 4
Current log sequence 4
SQL> exit