To update the firmware on a Spirent GSS6400 Record and Replay unit follow the procedure in DCS0044A
The latest firmware upgrade files are available to download for supported customers from the Spirent Support Website. Select the "Download" tab and then the GSS6400 product. Different download files exist for the different variants of GSS6400. Care must be taken to select the correct upgrade files for the variant you wish to upgrade, e.g. GPS only or GPS + GLONASS.