Now the best thing about this install is, it comes directly from Apple it doesn’t require Developer account it just requires you to install a profile from Apple’s website so it’s extremely simple to do.
Please note that you are going to install beta software so it is good to back up your data and files.in case anything happen to your Files while using the Beta Software.
So what you need to do is, on your iPhone or iPad go to beta.apple.com/profile and it will ask you to Sign with your Apple ID. If you’re already signed with your Apple ID it may take you to User Agreement choose agree and you will be redirected to Apple Beta Software Program Page.
Once the Page is loaded Select iOS tab and scroll down until you see download profile. Now go ahead and click on that download Profile button.it will take few seconds to load once it’s loaded it will ask you to install the iOS9 beta software profile you can go ahead and tap install.
Now you need to enter your PIN and agree to Apple’s Terms and Conditions click install to accept the terms and condition. Then it will ask you to restart your device.
Once your device is Restarted just go ahead and navigate the settings App and Tap on “General” in General Settings under About Section you will find “Software Update” tap on it and you’ll see a Download link to iOS 9 public Beta 1. Go ahead and click “download and install” and Enter your passcode one more time.
Once you agreed to the Terms and Conditions, your device will to start to download the iOS 9 Public Beta update. It is downloaded approximately in 15 minutes for me. However it will vary depending on your Internet connection Speed.
if you have any questions or problems feel free to ask me in the comments.