I don't need wb10 need w8
1. Download and install the latest version of the Windows Phone Recovery Tool (link) to your PC.
2. Launch the recovery tool and connect your phone using a USB cable. If your handset isn't detected, click My phone was not detected to force the application to rescan for and detect the phone.
3. In the next screen, click your phone, wait a few seconds, and you'll see your phone information and the software available for download to roll back to a previous operating system. To continue, click Reinstall software.
4. Next, the recovery tool will warn you to backup all your data, settings, and apps before continuing, as the rolling back process will delete all the previous data in your phone. Click Continue to proceed.
5. Now, the recovery tool will download the image from Microsoft's servers and replace Windows 10 Mobile that is currently on your phone. The process will take some time depending on your internet connection and the hardware in question.
After the tool completes the process, you will receive a message saying "Operation successfully completed". At this point, the phone will reboot and you will have to go through the Out-of-Box-Experience, like on any version of Windows. Then, you'll need to sign-in with your Microsoft account, select to restore your phone from backup (if this is something you prefer), and after a few additional questions, you'll be back to Windows Phone 8.1.
As you can see, Microsoft is making it very convenient to roll back to Windows Phone 8.1 from Windows 10 Mobile preview, and the company is starting to add support for non-Lumia devices, which is a big plus. So you don't have to spend time searching on the internet for different tools to go back to the previous operating system.
However, you have to remember that the operating system is far from complete. Not only will you find issues, but you might also stumble upon problems trying to roll back, as I hear many users reporting issues restoring Windows Phone 8.1 on their Lumia 930. As such, you should avoid using Windows 10 Mobile preview as your primary mobile operating system, at least until it hits RTM.