Apple offers towards download for free si installation (via Settings > General > software Update) The latest The final version of iOS, all usersWithout the need for them to make any special setup or other additional software to download.
The final version of iOS, which is available to all users, does not mean that it is the newest. It's the newest final version of iOS, though Apple permanently develops new versions that will be installed on all devices. Develop that GUI (UI - User Interface) and embedded applicationsNew functions and options.
Users who want to see and test the new iOS operating system before it can run in the final version can install on iPhone, iPad or iPod Touch betas provided by Apple through the program "Apple Beta Software Program". To do this, first of all we need to enlist your device (iPhone, iPad or iPod Touch) in the programmed beta.
Enroll iPhone or iPad in Apple Beta Software Program
Before you enroll the device in Apple Beta Software Program, we strongly recommend you to make a full backup via iTunes, Before following the tutorial below.
1. Go to page: https://beta.apple.com/sp/betaprogram/ and login with Apple ID.
2. Install profile because the devices can accept Beta versions of iOS.
After installing profile, the device will ask to restart. After the restart, go to Settings > General> Software Update, Get the latest beta version of iOS. Follow the steps download and install for the update to iOS Beta.
Currently, the latest Beta version of iOS is 9.3 3 iOS Public Beta.
Keep in mind that for any software version that is in the development stage "beta" may be some errors, poor functionality or loss of data (emails, pictures, documents, etc).