As probably a lot of people lately, I tried to get some of my devices to upgrade from Windows 7 or 8 to Windows 10. However, it was not easy, since I was not one of the first in the queue for MS obviously.
Well, anyway, there are some descriptions out there how to force your Windows to download the update (if you are not good with Google, take a look here for example: http://www.itechtics.com/download-internet-explorer-all-versions/). Anyway, if you are looking for the error code 80240020, that my Asus convertible displayed after I did this, most pages will just suggest to retrace that steps over and over again.
Now, after downloading about 2 GB of data (thanks, flatrate broadband!) for 3 times, I realized that a corrupt Download- or DataStore could not be the right solution. Therefore, I did some more research and stumbled over a page by Microsoft themselves, that suggested to add or create a registry key [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\WindowsUpdate\OSUpgrade\AllowOSUpgrade] to 0x0000001 (DWORD-type). The detailed solution can be found here: http://answers.microsoft.com/de-de/windows/forum/windows_10-windows_install/windows-10-fehlercode-80240020/5404b1f0-aa17-440e-848a-2dff0ace41c9?auth=1
As always, please be cautions when changing anything in the registry, because I cannot take any responsibility for any crashed computers :-).