On each pre-built website, in the the very top right corner or in menu, is the "Buy now" button. This feature is being used pretty often by Betheme users, thus it is included into each demo.
If Buy now is located along with other menu items, it means that you can edit it or remove under Appearance > Menus section.
But, if it looks more like a button and is located in the right top corner, just like on the image above, it is Action Button and you can manage it under Betheme > Header & Subheader > Extras section.
Moreover since update introducing Header Builder 2.0
, we place the button in the header template.