The Windows 10 automatic upgrade period will end on 29 July 2016. Before that, you may find that your computer has automatically upgraded to Windows 10, whether you wanted it to or not. If you find yourself in this situation, you can uninstall Windows 10, and revert back to your old operating system.
Option 1: If the upgrade occurred less than one month ago:
Go to: Settings > Update & security > Recovery
Note: This options preserves your documents and applications, but make sure you have a current backup before proceeding.
Option 2: If the upgrade occurred more than one month ago:
Go to: Settings > Update & security > Recovery > Reset this PC > Get started and look for Restore factory settings.
Option 3: If the above option is not available:
Go to Windows 10 Recovery Options, and follow the instructions under Go back to your previous version of Windows.
Note: For options 2 and 3, you will need to restore all your documents from a backup and reinstall all your applications from the original media (either DVDs or downloaded files).
Should you upgrade Windows? The short answer is: no. Your PC manufacturer tested your PC with the version of Windows it shipped with. Newer versions of Windows may or may not have been tested with your PC. Even if the manufacturer has tested the upgrade, it tested it starting with a fresh install or the original version, with no applications installed. The install of Windows on your PC has had months or years of changes made by new applications and device drivers. No testing can account for every possible permutation. That is why Windows upgrades so often end badly.