Update to IOS16 HOW?

Unlike iOS16 for iPhone, released 12th September (iOS 16.0.1 subsequently released 16th September), iPadOS16 is not due for release until sometime in October. 

However, iPadOS 15.7 was also released 12th September - incorporating essential security updates.

About the security content of iOS 15.7 and iPadOS 15.7 - Apple Support

While iOS16 is available for iPhone, iPhone owners have the option to just install iOS15.7 at this time.

More information about the forthcoming iPadOS16 can be found here:

iPadOS 16 Preview - Apple

ipadOS16 have not been released yet only ios16 for iphones have

you will have to wait until October it seems they are releasing it alongside the new macOS