So here are few steps that you may follow in ordere to recover your windows phone to 8.1 :-
1. All you need is only Windows device recovery tool installer, download here .
2. Install and open the software and connect your Windows phone 10 device that you want to be recovered. Your device is likely to be seemed like:
3. Tap on your detected device, next screen will be showing details of currently running operating system (i.e, Windows Phone 10)
4. Click on Install software, probably it should start downloading software of available version your Windows device,you may disconnect your phone while downloadig, and connect again while installing software on your device.
5. Once the software finished downloading, click on install , it may take around 15 minutes to install it on your device.
6. All done, click on finish, you successfully recovered your windows phone device from 10 to 8.1 . Next window seems to be appeared like:
7. Now feeling better, share to those also who are still stucked up to this problem, help them to roll back from windows 10 mobile preview.