This version is too old to remember (although it is the second FMP5 question I had today).
I do not think that you can. Since FMP 11 you can reset preferences to drop the password and then re-deploy.
If you have original installers and SN you can re-install server with the new password.
Uninstall FileMaker Server and reinstall it. See “Uninstalling FileMaker Server (Windows)” on
Mac OS 8.6 to 9.1
Quit FileMaker Server, then delete the files FileMaker Server 5.5 Prefs. This file is stored in the Preferences folder inside the System Folder.
Mac OS X
In the Preferences dialog, click the Restore Settings button, click OK, then restart FileMaker Server Config.
I am not sure this will reset the password but in the absence of better suggestions......