Compiled from helping in thousands of Upgrade failure cases, these steps work for everyone who tries them all for Upgrade from Windows 7, 8 or 8.1 and for version Updates of Windows 10 which are failing.
If confident your PC meets the Windows 10 Specifications and are aware of any listed unsupported OS features, then you can Install Windows 10 using the media creation tool choosing the first option to automate the Upgrade or Version Update.
However in the same amount of time it takes to download that option, you can choose the second option "Create Media for another PC" to make a bootable flash stick, or download an ISO to mount and run or burn DVD media, to keep for any needed repairs or reinstall, or can be run from Desktop to Upgrade. This media also gives an extra option to save nothing during the Upgrade which overcomes the Upgrade choking on corruption or bloated factory installs and gets as close to a superior Clean Install as an Upgrade can get.
If Media Creation Tool causes problems use the alternate download site at Microsoft TechBench. Make sure to download the correct version and bit-rate. Either Mount ISO to run Setup for Upgrade, or for bootable media burn ISO to DVD using Windows Image Burner, or create flash stick using Rufus choosing UEFI/GPT if you have a UEFI BIOS, or Legacy BIOS/MBR.
With bootable media you can even do the superior Clean Install Windows 10 which has zero complaints or reported issues in hundreds of thousands of uses and is the very best install of Windows one can have. Most tech enthusiasts will not even run a factory or Upgrade install but prefer this option for best performance.
To improve the Upgrade's chances turn off everything at Startup:
Type msconfig in Start Search box, right click System Configuration Utility to Run as Administrator. On tabs for Startup (via Task Manager) and Services (making sure to tick the box to Hide all MS Services ) disable everything. You don't need any except those which you prefer to have running when Windows starts and can enable them after the Upgrade. Repeat this step for the Startup folder by right clicking Start to access Run box, copy/paste shell:common startup press Enter to open Startup Folder.
Uninstall any anti-virus except Defender. Bloated AV's like Norton, McAfee, AVG and Avast need to run special Uninstallers (removal tools) for common antivirus software - ESET Knowledgebase.
Remove if present C:\windows.old folder following How to delete the Windows.old folder from Windows 10 - CNET. Restart PC several times.
I would also uninstall the buggy Windows Upgrade Assistant at Settings > Apps & Features.
Install CCleaner to Run Cleaner and on Registry tab Check for Issues and Fix Issues.
There are tips here for cleaning up and managing smaller drives including 32gb drives. Always create (on another PC) and run the media on cramped drives to save the huge amount of space the download and staging files takes:
Next type Command Prompt in Start Search, right click to Run as Admin, type:
sfc /scannow then press Enter.
There are reports that updating the BIOS or UEFI firmware is resolving some issues. Check the PC or motherboard's Support Downloads webpage to compare the latest with the presently installed one found in Windows Administrative Tools>System Information. Carefully follow the instructions for updating, often found in a ReadMe file in download, from the manufacturer's Support, or ask back here for help.
It’s highly recommended to Create a backup System Image of your existing OS before upgrading, which can be recovered using the booted Windows installation media, Repair CD or Recovery flash drive in 20 minutes. Type Backup in Start Search box, in Backup and Recovery Center choose Create a System Image. You can also use a more flexible imaging program like free Macrium Imaging which allows recovery to any partition position on the hard drive.
Remove any SD cards or external USB devices or peripherals.
If you prefer to run an in-place Upgrade the media must be opened from desktop or ISO double-clicked to Mount, then right click Setup file to Run as Administrator. If using Flash installer make sure it is not set first to boot in BIOS setup so it won't boot itself during restarts, or unplug it during restarts.
If you want to maximize chances for a successful Upgrade, then at the screen shown below choose the link to Change What to Keep and save nothing so the Upgrade has much less to choke on and has less problems after install.
Make sure your files are backed up separately and you have your program installers and any Product Keys for programs like Office. Everything that needs backing up is detailed in the Clean Install Wiki provided earlier. (You can save this option as a last resort.)
If it fails anyway then report back the exact error or failure behavior. Try running it again offline without the internet by disabling the network adapter or pulling the ethernet cable.
You can also try enabling the Built-in hidden Administrator used in emergencies, signing into it (blank password box) to run the Upgrade: http://www.top-password.com/blog/4-ways-to-enable-built-in-administrator-account-on-windows/
If Windows won't start see Troubleshooting Windows 8 and 10 Failure to Start - Microsoft Community. But there's a very good chance in any case that you have such a corrupted install it is not worth keeping, so I'd strongly consider at this point going from worst to best by closely following Clean Install Windows 10 which is the gold standard install in an OS where the quality of the install is everything.
Stick with built-in Defender in Windows 10 for best performance and adequate protection. If you get infected add the inexpensive Real Time protection from the best-in-class free malware scanner Malwarebytes. No one I’ve had do this has ever been infected again. I'd also periodically scan with free Malwarebytes and AdwCleaner scanners.
If the Upgrade repeatedly fails, you don't even want it and it is disrupting your PC then you can uninstall the Windows Upgrade Assistant and hide the Version Upgrade until it matures following this tutorial: https://www.howtogeek.com/224471/how-to-prevent-windows-10-from-automatically-downloading-updates/. There is media to reinstall or repair older versions at the TechBench site linked above.
Feel free to ask back any questions and keep us posted on how it goes.