Here I will let you know how to downgrade from your iOS 8 to iOS 7.1.2
If you still have iOS 7.1.2 IPSW you do not need to re download. Skip this discussion and proceed to the next point. But if not, here is a tutorial to detect and download it.
Here is the following steps to download 7.1.2.
1. Download software f0recast for windows or Mac OS X.
2. Open f0recast and connect your iPad, iPhone or iPod Touch to a computer.
4. It will direct you to open website icj.me.
5. Select iOS want to download (click on orange text)
6. Make sure on "Apple Signing Status" have green checklist. Or you can't restore your iOS.
7. If you have download iOS 7.1.2 then to next steps.
8. Open iTunes and backup you iPhone, or iPad.
9. Now, restore your iPhone or iPad, on Mac hold Option on keyboard or Shift on Windows then click Restore, select iOS you have been download. Make sure you are connect with internet.
10. Just wait for the restore process complete.