Have you heard about a feature on an upcoming iOS version, and can't wait to try it out?
Or maybe you have one of those iPhones which is really slow because of that intentional slowdown thing, and you want to get iOS 11.3 before it's actually released to the general public, thus allowing you to turn it off?
Or maybe you just like being ahead of the pack.
There is an incorrect belief that you must be a "paid developer" to get access to iOS/MacOS beta versions.
You can do it legally and easily FOR FREE with NO DEVELOPER ACCOUNT, by following these steps:
0) Back up your device
1) Make sure you have a valid Apple ID
2) Use the device you want to update to the beta, and go to
https://beta.apple.com
3) Click the
Sign Up button. Then enter your Apple ID and password as requested. Accept terms and conditions.
4) On the beta screen, click the OS you need (MacOS, iOS, tvOS)
5) Scroll down to where it says "Get Started", and then click on the link that says "enroll your device".
6) Scroll down to step #2, then follow instructions, and then proceed to step #3 and follow those instructions.
That's it!
If downloading iOS from the air (the only way to do it), it will probably take a few hours, so make sure your device is plugged in, turn off the auto-lock feature, and let it go.
One other note. Once it's finished downloading on iOS devices, it won't auto-install. You need to go to the Settings->General->Software Update screen and click "Install Update" at that point.
There is no limit to how many devices can download and use the beta.