The Screenboard Update API call requires the boardID to be part of the url path /screen/{boardId}

lib/api/screenboard.js

@@ -172,7 +172,7 @@ function update(boardId, boardTitle, widgets, options, callback) {
       params.body.read_only = options.readOnly;
   }
 
-  client.request("PUT", "/screen", params, callback);
+  client.request("PUT", util.format("/screen/%s", boardId), params, callback);
 }
 
 /*section: screenboard