Here's how you can download and install watchOS 3 on Apple Watch without an Apple Developer Program account.
Install watchOS 3 Without Developer Account - How To Guide
Apple's next-generation software lineup is as solid as it ever can be. New features are littered all over the place and many of them are waiting to be discovered. But the sad part right now is: you need an Apple Developer Program account to get in on the action right away. This goes for iOS 10, macOS, tvOS 10, and of course, watchOS 3. But luckily, you can bypass an Apple Developer Program account and install iOS 10 beta on your iOS device right now. Even better news is that you can do the same for watchOS 3 beta. We'll show you how.
Install watchOS 3 Tutorial
First and foremost, download and install iOS 10 beta on your iPhone. If you don't have an Apple Developer Account, you can still do so. Follow our guide posted here: Install iOS 10 Beta On Your iPhone Without A Developer Account. Once that's out of the way, you need to source yourself a watchOS 3 'Configuration profile.' You can do a quick search on the Web and find it in no time.
As soon as you download the watchOS 3 configuration profile on your iPhone, select 'Apple Watch' as the device on which you want to push the profile. Now head over to the Watch app, navigate to General > Software Update, and tap on Download and Install. The install watchOS 3 process will now begin, and will take a while to complete.
Please take note that before you begin with the installation process, charge your Apple Watch completely till the 100% mark. Also make sure your Apple Watch is placed on its magnetic charger throughout the process. Lastly, please refrain from using your iPhone during the installation process. Once the installation process is complete, set up your Apple Watch as you normally would and start using it.
It should be made abundantly clear that you cannot downgrade from watchOS 3 to watchOS 2. Once you've made the jump, there's no way you can go back. Until or unless you're willing to take your Apple Watch to Apple itself for assistance. There's no special cable or remedy for downgrading watchOS 3 at home, so be vary of this fact.
The final version of watchOS 3 will be available this fall, along with iOS 10, macOS and tvOS 10. But Apple will be pushing new updates every other week ensuring that progress has been made in the background.