How to replace default top frame with "buy now" button in a preview?

Hello! Here is my question. I want to replace default envato frame on the top of the page with my own demo navigation. It is intresting what are requirements and restrictions. I will be gratefull for any tips and guidelines!


most are custom built but there are some released for free such as

Github is where you will find some and via Google (looking at the old forums)


Thank you.