When you say "update an existing record in the table", I think you mean update the XML with the latest from the SharePoint list. Is that correct?
If so, you can do this with the RefreshSharePointListItems command. Generally, I'd suggest setting this command on the finishedLoading node of the QdabraRules data source -- when finishedLoading is true for the condition. That way, when the user opens the XML, the list items get updated.