The exact procedure does depend on the Extreme Management Center (XMC) / Netsight version. In the latest version we added some more options to configure that.
In general here are steps:
1. upload the firmware file to your XMC to proper directory
2. configure XMC Profile including CLI credentials
3. configure assign the XMC profile to your 3rd party device
4. assign the firmware transfer method to the 3rd party device
5. run the firmware upgrade