itunes - resume downloading of ios update

I am having slow internet connection and it usually gets disconnected after about 2-3 hours or while downloading large size files. I want to update to ios 10 in itunes for my iphone. While downloading the ios update halfway through, the internet connection stops and itunes application throws up message that there is no active internet connection and closes the window. Once again, I need to restart the download process. Is there a way to resume the downloading of the ios file which is about 2 GB? Otherwise i may never be able to download in my internet connection. Please provide if there are any other alternatives.
Thanks.