How are you trying? If your Win 7 or 8.1 has all it's updates, the upgrade will automatically download. You can also download the Microsoft Media Creation Tool, which will download and install Win10.

I was trying to download the update over my college wifi and it was stuck at "Starting download".

I ran a VPN ( psiphon ), and when the download started i paused and resumed after closing the VPN.

so sometimes it might be your network blocking the download.

Thanks for your help. but my computer suddenly decided to download W10.  Seems all is well.