membership - what is the shortcode to have member upgrade to premium subscription

I don't see a shortcode or a way for a user with a free membership to upgrade to another subscription level. The only short code is "[subscriptionform]" and if the user is already logged in and they go to the signup page there is only a message of "looks like your already logged in" or something like that. What do I need to do to get them to upgrade?
some better documentation would be nice, I'm just fumbling around here.
thanks