How can I replace one HB theme with another?

I purchased the Aegaeus theme from HB some time ago. In the process of working out some problems I’ve had with it, I noticed that the Highend theme seems more appropriate to my needs. Mine is a book author site. I will be presenting blogs about and displaying descriptions and visual images concerning my books and their subjects. No direct sales are involved.
I asked one of your tech help people if I could exchange my Aegaeus theme for the Highend theme and was told that I could. How do I go about switching themes?
(Incidentally, is importing the demo theme and customizing it the best way to proceed with building my site or is it better to use the included templates?
Thanks for you help,
Rob West