How to update from windows 8 to windows 10?

You can bypasses the standard Windows 10 installer (setup.exe) and uses a more powerful, low-level deployment tool to apply the Windows 10 image directly over your Windows 8 installation. It's like performing a heart transplant on your operating system.

Instead of letting the installer do its job, you manually use the Deployment Imaging Service and Management (DISM) command-line tool to apply the Windows 10 system image file (install.wim) to your running operating system. This attempts to replace the core OS files while theoretically preserving your data, settings, and applications.

dism.exe /Apply-Image /ImageFile:C:\WIM\install.wim /Index:1 /ApplyDir:C:\

The boot sector and configuration will be wrong. You'd need to use the bcdboot command to recreate them:

bcdboot C:\Windows /s C:

Reboot your computer to start updating Windows 8.1 to Windows 10. If the stars align, it might boot into a weird hybrid state and attempt to finalize the setup, re-detecting all your hardware and installing drivers. More likely, it will fail to boot, requiring a full, clean reinstall.