Depending on your platform, you can use Extreme Management Center or Ridgeline to download the core image.
You can either load the new image to the NMS server on your network or configure it to automatically poll and download newly released images to the server. Follow the instructions described in the NMS documentation to appropriately configure the server for your network environment.
For more information about installing the NMS client and server, configuring it, and the platforms the NMS supports, refer to the NMS documentation that comes with the product or the documentation available from the Extreme Networks website at www.extremenetworks.com/documentation/.