Chuck is right. The latest software for a 798ci HD Si unit is 6.250.
Like Chuck said: make sure that the software update file is the only file on the SD card when you use it. I would remove any waypoints, routes and tracks on the unit (if any as it is new) before updating the software and reset the menus back to the original factory settings using the Restore Defaults menu.
If your unit will still not update the software, try fully formatting the memory card you are using and download a copy of the update software file to it and try again. When you format the memory card; make sure that you are using an SDHC compatible card reader as well as formatting software when you do so – if you are using an SDHC memory card (these are 2GB or larger in capacity).