Resetting an unknown AIX root password

The following procedure describes how to recover access to root privileges when the system's root password is unavailable or unknown.

The following procedure requires some system downtime. If possible, schedule your downtime when it least impacts your workload to protect yourself from a possible loss of data or functionality.

1. Insert the product media for the same version and level as the current installation into the appropriate drive.
2. Power on the machine.
3. When the screen of icons appears, or when you hear a double beep, press the F1 key repeatedly until the System Management Services menu appears.
4. Select Multiboot.
5. Select Install From.
6. Select the device that holds the product media and then select Install.
7. Select the AIX version icon.
8. Define your current system as the system console by pressing the F1 key and then press Enter.
9. Select the number of your preferred language and press Enter.
10. Choose Start Maintenance Mode for System Recovery by typing 3 and press Enter.
11. Select Access a Root Volume Group. A message displays explaining that you will not be able to return to the Installation menus without rebooting if you change the root volume group at this point.
12. Type 0 and press Enter.
13. Type the number of the appropriate volume group from the list and press Enter.
14. Select Access this Volume Group and start a shell by typing 1 and press Enter.
15. At the # (number sign) prompt, type the passwd command at the command line prompt to reset the root password. For example:

    # passwd
    Changing password for "root"
    root's New password: 
    Enter the new password again:

16. To write everything from the buffer to the hard disk and reboot the system, type the following:

    sync;sync;sync;reboot

When the login screen appears, the password you set in step 15 should now permit access to root privileges.

For more information refer below link

http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.baseadmn/doc/baseadmndita/HT_baseadmn_recoverrootpswd.htm