Microsoft has announced this a known issue. It seems that you are getting stuck at 0% or any other percentage, but the build keeps downloading. Some Windows 10 users reported that it took their almost two hours to download the build. So you can ignore it and just be patient. Then the build should download fine.
If you can’t wait so long, try below method.
1. Click Start button.
2. Type “services” in the search box and click on Services on the pop-up menu.
3. In Services dialog box, find and double-click on Windows Update.
4. Under Service status, click Stop button. Then click OK button.
5. Reboot your PC.
6. Launch Windows Update and check for updates again.
7. Download the build again. It should download quickly.