Step 1: Connect your iDevice with iTunes via USB.
Step 2: Restore your iPhone, iPad or iPod touch to iOS 7.1.1 via iTunes, then setup it as a new iPhone
Step 4: Now your are requested to hold SHIFT and left click on “Check for Updates” on iTunes, then select iOS 8 beta 1 which you downloaded on Step 3, wait for few minutes and enjoy iOS 8
You can download iOS 8 beta firmware from the following link:
Download iOS 8 Beta Firmwares
Ref: MMAS