Recently an update for iOS devices (i.e. 9.2.1 for iPhone and iPad) was released and I clicked on the “Update” button on iTunes. The operating system was downloading smoothly and when only 1 GB was left to be downloaded (usually an update is around 2.22 GB), I got an error message similar to the one given below:
Figure 1 : Error 9006 in iTunes 
No big deal, right? My internet connection was working perfectly but I restarted my router and performed the updating process again. Again the error message popped up when the remaining volume of the update was 1 GB. What a waste of time and data?
Without being disheartened, I did some research on the web and found out this issue could be a result of third party security software installed in your computer (Mac or PC). For example, I have AVG AntiVirus installed in my Mac.
Figure 2 : Some popular antivirus software 
The first step was to turn “Realtime Protection” off.
Figure 3 : Realtime Protection in AVG AntiVirus for Mac 
Check if the issue has been resolved. However, this was not helpful, so AVG was uninstalled.
Finally, the update was successful. I know that uninstalling and reinstalling your antivirus software could be a bit inefficient but it seems to be the only way to get the problem solved.
If the issue still persists, try connecting the USB data cable, which was provided with the iOS device, to a different USB port of the computer and download the software update via iTunes. I know this sounds absurd but it would surely fix it.
Alternatively, try to update the iOS OTA (Over-the-Air) by following the procedure given below:
- In your iOS device, tap on “Settings”.
- Next, tap on “General” and then on “Software Update”.
Figure 4 : Updating iOS via OTA 
Hope this helps.
 5 Ways To Fix IPhone Error 9006 – TechnoBezz. 2016. 5 Ways To Fix IPhone Error 9006 – TechnoBezz. [ONLINE] Available at: http://www.technobezz.com/5-ways-fix-iphone-error-9006/. [Accessed 27 February 2016].
 Computer Flu – Anti Virus Setup – Blaby, Enderby, Countesthorpe, Narborough, Whetstone, Huncote, Broughton Astley, Cosby, Wigston. 2016. Computer Flu – Anti Virus Setup – Blaby, Enderby, Countesthorpe, Narborough, Whetstone, Huncote, Broughton Astley, Cosby, Wigston. [ONLINE] Available at: http://www.computerflu.com/2-anti-virus-setup.htm. [Accessed 27 February 2016].
 AVG enters Mac anti-malware market with AVG AntiVirus for Mac Free. 2016. AVG enters Mac anti-malware market with AVG AntiVirus for Mac Free. [ONLINE] Available at: http://betanews.com/2013/12/13/avg-enters-mac-anti-malware-market-with-avg-antivirus-for-mac-free/. [Accessed 27 February 2016].
 How to Upgrade to iOS 9 Using OTA. 2016. How to Upgrade to iOS 9 Using OTA. [ONLINE] Available at: https://www.iskysoft.com/article/upgrade-to-ios-9-ota.html. [Accessed 27 February 2016].