The only way to revert back a previous version of Windows is through flashing. You will have to download and launch Windows Phone Recovery Tool. Connect the phone and simply click on Reinstall software. At this point, the software will check all the starting conditions and provide the necessary steps. Your phone needs to be recharged to minimum 85 % for the process to start. However, in some cases, if Windows 10 is the latest stable OS, then the software will install that version, but based on my experience, these tools will install a previous version.
Alternatively, study the Microsoft answer: http://windows.microsoft.com/en-us/windows-10/going-back-to-windows-7-or-windows-81
