1. First, You need to back up the configuration.
2. Change the name from .dat to .tgz format of your saved backup file
3. Install Total Commander
4. Edit Total Commander setting to use notepad++ as Editor.
5. Open the backup .tgz file:
6. Edit the system.conf file.
7. Change the admin name and save the file that you will find in config system admin user:
edit "admin1", for example
and change password:
set password "new_password"
8. Repack the system.conf by closing the edit notepad app
9. Navigate up to the .tgz root file
10. Repack the .tgz file
11. Change back the config backup filename from .tgz to .dat
12. Reload the backup file in your FortiManager or FortiAnalyzer: