Download the IPSW from here: https://ipsw.me/
Manually install through restore. Example: http://osxdaily.com/2010/11/23/how-to-use-ipsw-files/ (Click restore instead of update button for a full wipe)
Manually install through restore. Example: http://osxdaily.com/2010/11/23/how-to-use-ipsw-files/ (Click restore instead of update button for a full wipe)
...and iTunes is unusable during this cycle.