Apple released iOS 8 just a couple of weeks ago and everyone has been going berserk about getting their hands on its beta which has only been made available by Apple for registered developers. But there is a solution to every problem and here we are with the solution to this little issue.
Note: iOS 8 requires iPhone 4S and above. Please do not try this if you do not have a compatible device. For the list of compatible devices, click here.
Step 1: Connect your device to iTunes and back it up so that in case of any emergency you don't have to lose your data.
Step 2: Download and do a fresh Install (or Restore) of iOS 7.1.1 on your iDevice. You can download the respective IPSW for your iDevice here. You can do that by clicking on Restore button holding Shift (for Windows users) or Alt (for Macintosh Users) and pointing to the IPSW file and then select Set up as new device.
Step 3: Now you need to download iOS 8 Beta IPSW for your respected device. Here are the links to iOS 8 Beta 1 IPSW file.
Step 4: Connect your iDevice to your PC and go to iTunes. Again click on Restore button holding Shift (for Windows users) or Alt (for Macintosh Users) and point to the iOS 8 Beta IPSW file you downloaded in the last step.
Et Voila. Your iDevice is now running on iOS 8 Beta.
So that you don't bump into any roadblocks or any confusion in the process, here is a video tutorial to help you out.