1-The first thing to do is get the version of the iOS operating system that would settle on your device. As we say, Apple does not offer the possibility of returning to an earlier version once you've updated, so you'll have to search the Internet until you find a link that allows you to download the operating system by an external way to the apple company .
2-Once you already have it, the next step to uninstall iOS 8 to return to the previous version is to connect your iPhone or iPad to the computer you have the iTunes program, through the USB cable.
3-Then open the iTunes program, if not opened automatically when you connect your device and select the iPhone or iPad, then pressing the Restore option.
4-Now is when you should select the file you downloaded from the Internet and containing the version of the operating system that interests you install on your Apple device.
5-Sometimes everything goes well and your iPhone or iPad is restored smoothly and get uninstall iOS 8 to return to the previous version. However, iTunes may not recognize the file and you get to lock your phone iPhone or iPad.
6-If you think this will last and iTunes are closed continuously every time you connect the device, try to solve the problem with the next solution. Connect your device to another computer with iTunes and restore it to the version of the operating system that offers Apple officially.