To update: 

1) Connect the X873 live on a USB port (without going through a hub) 
2) Launch the MPTool tool (v1.1.1.1) provided
3) The software requests to be unlocked via the button represented by a padlock located in the center at the top of the window, 
4) The code requested is: asmedia
5) Click on the FW browser button to search for the file: AS_PCIE_200811_81_00_00.bin
6) Click on the START button represented by a blue PLAY type circle located at the bottom left of the window.