Apple does not allow installation of beta versions on iOS devices that are not registered as developers. However, there is a way around that.This is how to install iOS 9 without a developer account.
Follow this guide on installing iOS 9 beta.
Step 1: Download the leaked iOS 9 version.
Step 2: Get your UDID registered for free.
Step 3: Connect your iPhone, iPad or iPod touch to your computer and start iTunes.
Step 4: Press “shift+right click” (Windows) or“Alt+right click” (Mac) on the update button and select the file you downloaded in step 1.
Step 5: It will now start updating your iOS device to iOS 9.
Step 6: Once that is done, your iPhone, iPad or iPod touch will have iOS 9 installed on it.