After updating my gadgets with iOS 9 I\'ve tried to update Xcode to Xcode 7 on my Mac through the App Store, but updating is too slow while speedtest.net shows that my download
I had the same issue, the update ran forever and never finished. I uninstalled Xcode and when i tried to install it again it said I needed the OS X 10.12 at least.
So i upgraded OS X and then installed Xcode 8.3
I wish the update told me that before i wasted 2 days trying to figure it out.. Hope it helps!