Try this, it works for me in both cases

If you are OTA update
Changing DNS in iOS

1. Tap on Settings, tap on “Wi-Fi”, and tap on the blue arrow next to the router name
2. Under “DHCP” tab tap on “DNS” and replace with: “208.67.222.222″ for OpenDNS
3. Tap the Back button and attempt to use OTA again

Or if you update using iTunes
Change DNS in OS X

1. Open System Preferences from the Apple menu
2. Click on “Network” and then click on ‘Advanced’ in the lower right corner
3. Click the “DNS” tab and then add a new DNS server by clicking the “+” icon, adding “208.67.222.222″ for OpenDNS
4. Drag the newly added DNS server to the top of the list, click “OK” then close out of System Preferences
5. On a Mac you may need to follow this up with flushing DNS cache, so open the Terminal and type the following:
dscacheutil -flushcache