Is your device automatically upgraded from Windows 7 or 8.1 to Windows 10 (that’s quite possible considering Microsoft going hard at users to move them to new Windows version, you can prevent that from happening by using a utility like Never 10). Within 30 days, you can roll back to a previous version of Windows from within Recovery in Update & Security of Settings app.
What if your upgraded Windows 10 fails to boot and if you want to get back to Windows 7 or 8, then you need to fall back on free utility from Neosmart called Windows 10 Rollback Utility, which also disables automatic updating to Windows 10 like Never 10, once the rollback process completes.
Windows 10 Rollback utility is based on Neosmart’s Easy Recovery Essentials for Windows. If your PC is not currently working after the upgrade, download the utility on another system that is having an internet connection, to USB drive or CD or DVD, use it and mount as the first boot option on your PC in BIOS.
Run it (check the instructions shown in this video), once your Windows 10 and Windows 7 partitions are shown, select the latter, wait for some time for the rollback process to complete, restart your computer once the screen shows the rollback is complete message.
Note: Since Windows 10 automatically removes Previous Windows installation files after 30 days, you need to be using this tool within a month period or so after the upgrade, for the recovery to be successful.
You need to provide your email address to download the ISO file, which is close to 200 MB.
Now read about similar tool Easeus offers: Downgrade to Windows 7/8 from Windows 10 using EaseUs System GoBack Free