To reset the known U/P credentials for ALL devices
  Settings Tab -> Settings -> Config Collector
  Click the "Reset All Authentication Info" button

To reset the known U/P credentials for a single device
  Open the Device Viewer for the particular device
  Select Config Files -> Settings
  On this Screen, click the "Reset Authentication Info" button


This will force NetMRI to attempt to guess the credentials for this device, or all devices, again, based on the credential input to the Global list - Settings Tab -> Settings -> Config Collector