This is a fix for 80240020 error. This is the error I encountered.
This error means one of the download files is corrupted.
1. Go into "C:\Windows\SoftwareDistribution\Download" and delete everything in that folder.
2. Now, run the command prompt as an administrator. Type in "wuauclt.exe /updatenow".
3. Go to your Control Panel > Windows Update and your Windows 10 will start downloading again.
The deleting of the files will not damage your operating system!
Here is the source to the solution
Hope this helps and saves some time searching Google.