Chapter 10 Install and System Administration for FortiOS 5.0 : Basic Administration : Administrators : Security Precautions : Change the admin username and password
  
Change the admin username and password
The default super administrator user name, admin, is a very standard, making it easy for someone with malicious intent to determine or guess. Having the correct user name is one half of the key to the FortiGate unit being compromised and so the default name should be changed.
To do this, you need to create another super user with full access and log in as that user. Then go to System > Admin > Administrator, select the admin account, and select Edit to change the user name.See Also
Security Precautions
 
Disable admin services
SSH login time out
Administrator lockout
Idle time-out
Administrative ports
Change the admin username and password
Segregated administrative roles