This article has been copied from http://download.cnet.com/blog/download-blog/stop-or-roll-back-a-windows-10-update/
As long as the update to Windows 10 has happened within the last 30 days, you can revert surprisingly easily. We recommend making a backup copy of Windows 10 before you get started, in the unlikely event that something goes wrong. Also, be aware that any apps you've installed since the Windows 10 update will need to be reinstalled after you revert to Windows 7 or 8.1.
Tap your Windows key and type "settings" (without the quotes) to get a link to the Settings tool. Click that and double-click Update & Security, which should be listed at the end. Select Recovery in the left-hand pane. You'll see a list of recovery options, including Go Back to Windows 7 (or Go Back to Windows 8.1, if that was your most recent version). Click the Get Started button. The process duration depends on the speed of your PC -- it could take a half hour or more.
If you're open to the idea of using Windows 10 but don't want to take the leap yet, you'll need to change a setting in Windows Update to prevent the upgrade from happening automatically. In Windows 7 or 8.1, press the Windows key and type "Windows update" (without the quotes). Click that option in the search results. If it's not in your results, type "control panel" to open the Control Panel, which contains the Windows Update tool.
Open the tool and click the Change Settings link in the left-hand pane. Then uncheck the box next to "Give me recommended updates the same way I receive important updates." Click the OK button at lower right to save your changes and close the window.
If you're using Windows 7 or 8.1 and you don't want Microsoft to send you notifications to update to Windows 10, you'll need a third-party tool called GWX Control Panel. GWX is the shorthand for Get Windows 10, which is the title of Microsoft's initiative to distribute the OS. There are a number of tools that block GWX, but GWX Control Panel is the most popular one.
This tool contains a number of functions, but the most important ones are the top two buttons. Disabling Microsoft's GWX 10 app will stop its pop-up notifications and remove its icon from your system tray. Or you can click Prevent Windows 10 Upgrades to block automatic updates, similarly to what we described above.
There's also a Delete Windows 10 Download Folders options. To facilitate Windows 10 installation, Microsoft sends about 3.5GB of files in the background. As far as we know, the company is not in the habit of asking permission to do this, or in the habit of mentioning it after the fact; it kind of just happens. GWX Control Panel will show you how much space the folder is currently taking up.
Windows might stop behaving if you disable Microsoft's Get Windows 10 hooks, but you can usually resolve this by clicking the GWX Control Panel button labeled Clear Windows Update Cache. We recommend clicking that button regardless, to reduce the possibility of lingering issues with Get Windows 10.
When you're done making changes in GWX Control Panel, just click the X in the upper right-hand corner to close the tool.