You will need to go into the Matter Options and click the 'Clear' button next to the name of your Workspace. When you've cleared it, you'll get a notification at the bottom of your screen confirming the mapping has been removed.
To map a new workspace, navigate back to the NetDocs tab within the Matter and select the correct Cabinet.
NOTE: Check with your firm Admin that your Client-Id/Matter-Id Format is compatible with NetDocuments.