How to change the name of a button from another page ?

Hi guys , I want to create a picker with Rg.Plugin popup . I created the popup page as listview . So the popup page will open if you click the button that is in Register Page...What I want to do now is when user click 1 item in listview the button name to be changed!

I have error because button name don't show in my Popup Page , how can I do that ?