I am trying to help a relative telephonically who is not computer literate to install Win10
Might be a good idea to look in the root of C:\ and see if
$windows ~bt is present. It's a hidden file so you might have to change your folder options view tab "Show hidden files, folders and drives".
IF so, you may want to delete it and that will force the process to download the installation files again.
IF it is continuously failing then the contents of that folder may be corrupt. IT would probably be a good idea to re-download the installation files anyway just to make sure you are getting the latest version.
Failing that you may need to create your own physical installation media.
https://www.microsoft.com/en-us/software-download/techbench
Choose Windows 10, the first item in the first drop down (not single language at the bottom)
* see note at bottom.
Next choose your language and your bit version (32 or 64 bit to match your system architecture or in the case of an upgrade, to match your currently installed version of Windows).
That will provide an ISO that will upgrade, repair (or clean install) either or both Windows 10 Pro and Home.
Once you have downloaded the ISO you can use ImgBurn to burn it to a DVD
http://imgburn.com/index.php?act=download
OR
Rufus to burn it to a USB ThumbDrive
http://rufus.akeo.ie/
When you've created the installation media then you simply launch setup.exe from the media from within your current version of Windows to perform the in-place upgrade.
In the case of upgrading from Windows 8.1 as seems to be your situation.... You can simply double click the ISO file and that will mount it as a virtual drive, where you can select and run setup.exe.
*NOTE: In some cases, we've learned that some people have the "Single Language" version installed. In which case you would need that download. It's important that you match the ISO version that you download with your installation.
I would also suggest that in addition to a critical data file backup you might want to consider performing a disk image.
https://www.windows10forums.com/threads/please-for-your-own-peace-of-mind.794/
It never hurts to have a strong fall back position in cases things don't go as planned.