I was feeling really good with the update downloading in 15 minutes, extracting and backing up working---until I received this error. I tried to restart the PC and restart the itunes to do the update again-and had the same message. I called Apple support and the kind lady told me that this error has coming up frequently because of the server being bogged down. Her recommendation is to try the update later when it might not be so congested.
Hope this helps anybody out. I asked if I should do anything else to my router, security updates etc--she told me no.