This article is a how to on downgrading your iPhone, iPod Touch or iPad to iOS 7.1.2 from iOS 8.
Maybe you’ve just upgraded from an iPhone 4S and it’s not performing adequately. Maybe you just don’t like the changes. For whatever reason you may have (including jailbreaking) you may want to undo your decision to upgrade and want to downgrade.
If you’ve checked that page and your firmware still is being signed, you’re OK to continue to the next step.
Next, download your IPSW file. This will be different for each model of device, and you can download the right one by going to this page. Make sure you get the right model or things could go wrong later.
After the IPSW has downloaded (it may take some time depending on your internet connection), open up iTunes and connect your device. Then hold the left ‘SHIFT’ (PC) or ‘Alt/Option’ (Mac) key and click on ‘restore’. Then select the file you downloaded earlier.
Note: If you have Find My iPhone enabled, it will prompt you to turn it off before restoring.
After this, the device will go through the process of restoring and then you should be good to go.