Run SQL*PLUS using Run SQL Command Line in Oracle Database 11g
Connect to Oracle Database as SYSDBA as follows:

SQL>connect as sysdba
It will ask you to enter username. Enter SYS as username. change_on_install as   Password
When prompted for password, enter , which is the password for SYS account.

SQL> ALTER USER system IDENTIFIED BY oracle;
The above command changes password of SYSTEM account to oracle. Be careful passwords are case-sensitive.
That's all you have to do to change password for SYSTEM account when you forget the password.
Here is the snapshot of the SQL*PLUS screen, which shows all required steps.