If you’re a developer with a device already running a previous beta version of iOS 11, you can upgrade to iOS 11 beta 6 (build number 15J5360b) over the air using the Software Update mechanism in the Settings app.
watchOS 4 beta 6
Along with iOS 11 beta 6, Apple today seeded watchOS 4 beta 6 (build number 15R5357b) to the registered developers and members of its developer program.
If you’re a developer and would like to install watchOS 4 beta 6 on your Apple Watch over the air, tap My Watch → General → Software Update in the Watch app on your paired iPhone.
Non-developers are not permitted to test the software because the company’s official public beta-testing program currently does not support the watchOS platform.
macOS High Sierra 10.13 beta 6
An over-the-air macOS High Sierra beta 6 (build number 17A344b) download is available through the Mac App Store’s Updates tab. The full installer can be grabbed from Apple’s Dev Center portal for developers.