If you mean change how they look then you have to edit them manually, either by hand coding, or using Dreamweaver etc. Lots of guys on here edit templates all the time, Im sure they can do the work for you if you need it.
I think the OP refers to changing the template, as using a different template. I believe you will have to download a template you wish to use then replace each file overwriting any files that are located on your FTP so make sure you download the correct version. Regards, Meti
as you are using the free version you must go into FTP and delete the old files. If you use 3.2 you can do it from the admin.
As title says He needs to upload all .tpl files into template folder and images files into image folders (most free templates includes instruction alongwith zip file) so go through it.