If you've upgraded your system to Windows 10 you have most likely done through Windows Update, so you can uninstall the preview by undoing the action back. For this you need to open Windows Updates on your system and check the last installed updates which were done. You have to look for the Update for Microsoft Windows (KB2990214) and then uninstall it.
Once uninstall procedure is over, you reboot your computer and you'll get your version of Windows restored.
You also need to Remove Windows 10 Technical Preview From Multiple Boot
Now to remove Windows 10 version from a multi-boot installation machine, you have to go to the Advanced Settings at boot and follow these steps:
1. Re-Start the system and Choose an operating system screen.
2. Select the Change defaults or else choose the other options.
2. Then Choose an option screen and select Troubleshoot.
3. In the Troubleshoot screen, select the option "Uninstall Preview updates" and you can get rid of Windows 10 and get back to the earlier version of Windows you had.