Apple has just released iOS 10.3 beta 1 for developers. If you have a previous iOS 10 beta installed, you can head to Settings > General > Software Updates and get started downloading. Note that this update moves your device's file system over to the Apple File System, making downgrading difficult. If you were waiting for iOS 10.3 to get started with beta testing, keep reading and we'll help you get set up.

iOS 10.2 is on its way with new features, several that require lots of third-party developer input. As such, the company is releasing developer betas so that software engineers can get a head start on making great software for you.

How to backup your iDevice on 3uTools?

Before you begin, make sure you back up your iPhone or iPad. While the beta process is fairly straight forward, any time you make any significant change to your device you risk problems. And with your personal data, it's always — always! — better to be safe than sorry. Even if you back up every night, you'll want to make sure it's absolutely up-to-date.

Plug iDevice into your PC.

Launch 3uTools.

Click on Toolsbox > Backup & Restore > Backup iDevice, and click Backup.

Wait for backup complete.

Make sure backup is set to This Computer. 

Please check this tutorials: What's Backup & Restore?

Now you could use 3uTools to update your iDevice.