iOS 8.4 long download

I am trying to reset my iPhone to factory settings to sell it, but it won't let me do that unless I download and install iOS 8.4. The problem is that it says it'll take 8 HOURS to download the 1.79 GB file. I can download a 2 GB movie (from iTunes!) in 20 minutes. Why is this taking so long, and is there a way to reset to factory settings without the update?