I have searched high and low and cant seem to find how to remove the menu button labled "Buy Now". Where can I remove/change this please?
What you are referring to is the action button. Please go to Betheme -> Theme options -> header &subheader -> extras and change the action button. See here:
We sometimes also add the 'BUY NOW' button as the last menu item, be sure to go to Appearance -> Menus and check there as well.
Also, if the demo uses Header Builder, you will find this button in the header template.