How to upgrade my iOS 15.6 to 16.0

You wait until its released, and then you update as normal in Settings->General->Software

iOS 16 is currently in testing phase and not available to update to unless you are part of the Beta testing.

iOS 16 won't be available until sometime this fall(the spring in the Southern Hemisphere). If you're involved with one of Apple's beta programs, follow the instructions they gave you.

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