In iTunes, you can install a beta operating system from a restore image that you download from Downloads. Beta software is intended only for installation on development devices registered with your Apple Developer Program membership. Registered development devices will be able to upgrade to future betas and the final operating system software.
Launch iTunes on your Mac.
For iOS and tvOS apps, connect a device to your Mac. For watchOS apps, connect an iPhone paired with an Apple Watch.
In the upper-left corner, select the device.
Option-click the Restore button, select the software restore image (a file with a .ipsw filename extension), and click Open.
When installation is complete, activate the device and use iTunes to restore its contents.
WARNING: The beta version you install must be the same as or newer than the version currently installed on the device. Attempting to install an older software image will fail. After a failed installation attempt, you must reinstall the last successfully installed version or a newer version.