You can either edit your unattend XML file that is associated with your image to change the administrator password (OPTION A) or you can create an application that calls a batch script to do it (OPTION B).  You can also use PowerShell, GPO, or a host of other ways.  

OPTION A: Open the Windows System Image Manager located on your smartImager server at "{Install Drive}Program Files (x86)Windows Kits10Assessment and Deployment KitDeployment ToolsWSIMimgmgr.exe"
Edit or create a new unattend xml file using the new administrator password.
Save the unattend xml file - then associate the new unattend file with you image in the smartImager console (Components | Images).
Save the changes to your image settings in the smartImager console (Components | Images).
Publish your settings.

OPTION B: Create a folder named "Scripts" on your smartImager server at "{Install Drive}Program FilesSirckssmartImagerRepositoriesApplications"
Create a batch file named "ChangeAdminPassword.bat" in your ".Program FilesSirckssmartImagerRepositoriesApplicationsScripts" directory.
The ChangeAdminPassword.bat contents should read: net user  administrator  newpassword
(change "newpassword" to your new password)
Create a new application in smartImager (Components | Applications) with the following settings:
Name: Change Admin Password
Folder Path: C:Program FilesSirckssmartImagerRepositoriesApplicationsScripts
Install Command: C:AppsScriptsChangeAdminPassword.bat
Install Type: Install After Imaging - After Domain Join (IMPORTANT)
Retry: Do not retry installation
Cleanup: Delete all files after installation (IMPORTANT)
Required: Continue imaging after failure
Reboot: Only Upon Error
Exit Codes: blank
Add the new application to an application pack, then publish your settings.