There is no supported mechanism to reset the password if it is forgotten.
From the user guide - How Do I Force Removal Of The Virtual Appliance?
1. Go to https://<vcenter_serverIPAddress>/mob
2. Click Content.
3. Click ExtensionManager.
4.Click UnregisterExtension.
5.Enter the extension key to unregister com.dell.plugin.Dell Management Plug-in, then click Invoke method.
6 In the vSphere client, power off the Dell Management Plug-in and delete it.