Re: changing the template
You will upload all your files accordingly, via FTP to your site. You, then go to Admin>Tools>Template Selection. You then, click on the "Edit" button you'll see there. If you've uploaded your files in the correct places, you will see the templates name in the dropdown. Select it, click "Update", and your done!
Being new to Zen-Cart, you'll find a LOT of your questions can be answered by clicking on "Tutorials/FAQ's" at the top of this site. One very nice Tutorial on this particular subject can be found HERE.
Hope this helps.
Teach them to shop and they will shop today;
Teach them to Zen and they will OWN a shop tomorrow!