Such a function doesn’t currently exist and I am not sure where it would appear on the frontend. The only thing I could think of would be some sort of widget that could be placed in a widget area that would appear when a user is logged in.
Even doing that would require a form submission of some sort although I suppose a link with some sort of action embedded in it could be created. Either way, it isn’t a simple operation – what would you envision happening after the user changes their subscription?
Currently the user can change their subscription status from their own profile page as long as the ability to do so is enabled. Since the user’s profile is typically available from the WordPress bar at the top of the page, would using this work?