Hi,

all firmwares for GHL devices are part of a GHL Control Center installation, which you can get from the Download-page.
Perform the update via USB. You'll need a system that can operate Windows to use GHL Control Center for the firmware update.
Mac users can take Parallels and run Windows from there.

Before updating, always save the sensor data and settings, so you can restore the data, if the firmware update must load factory defaults afterwards.