My Model S stalled during an update a few months ago. I went to my close-by service center for help. Long story short, it turned out to be caused by a bad file on the USB memory card I was using for music. Once the USB stick was unplugged, the software installed OK.
Having said that, I forgot to unplug my USB stick last night when I initiated a software update and it installed just fine. (I had deleted the bad file from my music collection and bought a new USB device since the previous installation failure.)
So, do you have a USB memory card or stick? If so, you might try removing it in case that might help.