Althoug this new feature is very helpful, but there are some conditions that must be followed:
When you upgrade your windows 7/8.1 to Windows 10, previous windows files and folders are automatically saved in the C:\Windows.old folder. And this folder will be deleted automatically after 1 month. So in this conclusion, you can only restore your windows 7/8.1 within span of 1 month after upgrading to windows 10. And please also note that if you run Disk Cleanup make sure the 'Previous Windows Installer (s)' option is unchecked. If checked, windows will automatically delete all files and directories that exist in the C:\windows.old folder, that result in Win 7/8.1 cannot restore again.
Restoring Previous Version Of Windows
1. Go to settings by clicking 'Start' and then click 'Update and Scurity'.
2. Click Recovery bar, there you will see the option 'Go Back to Windows 7 or 8.1'.
3. Click 'Go Back to Windows 7 or 8.1', when you are asked about the reason why to go back to old windows, just select the most relevant answer and click 'Next'.
4. After the question above, you will be asked some more questions, but here we through by click 'Next'.
5. The process to restoring Windows 7 / 8.1 will start, At this point your computer will automatically restart and you will see a notification 'Restoring Your Previous Version Of Windows', after restart process, you will see the results.
Note: if you cannot log in to Windows 10, you can use following methods:
1. Click the Power icon in screen login, hold down Shift key and click Restart, After PC reboot 'Advance Startup Screen' will open automaticaly.
2. Click troubleshoot then 'Advance Option' and click on 'Go Back To The Previous Build' option.