How to install iOS 8 on your device

Apple's iOS 8
1. Register the device UDID with IMZDL (make sure you receive a confirmation).
2. Update iTunes to the latest version (if youre not on it).
3. Connect the iPhone to a computer via USB and back it up via iTunes.
4. Download the iOS 8 Beta for your iPhone / iPad model.
You can do this via the official Apple developers portal (external link) or via IMZDL (external link)
5. Open iTunes and upgrade to iOS 8 Beta by clicking on Restore.
Hold alt/option on Mac when you click Restore
Hold Shift On Windows when you click Restore.
6. Find the .ipsw file you downloaded on your computer and select it.
7. Finish the installation.
8. You can also restore your device from the backed up state, if you want to.

OR

Just watch the video and follow the steps