With SqlPlus
You must connect with the SYSDBA privilege. If you don't remember the sys password, you can perform a OS authentication. Check this article for more information : Oracle Database - How to connect SYS as SYSDBA without password
C:\Documents and Settings\gerardnico>sqlplus / as sysdba
SQL*Plus: Release 10.2.0.4.0 - Production on Wed Jun 10 11:11:51 2009
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Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
SQL> show user
USER is "SYS"
SQL> connect sys/QS2007 as sysdba
Connected.
SQL> passw sys
Changing password for sys
Old password:
New password:
Retype new password:
Password changed
SQL>
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Support
If you don't give the SYS password before to change it, you may have the error ORA-28008.
SQL> connect / as sysdba
Connected.
SQL> passw sys
Changing password for sys
Old password:
New password:
Retype new password:
ERROR:
ORA-28008: invalid old password
Password unchanged