Generally there are two ways to upload themes to your Moodle site. The first is highlighted here by Dave Mozealous (@dmozealous) and is uploading to the theme directory on your Moodle installation using FTP.
Link to Dave’s helpful post: http://www.mozealous.com/find-change-and-install-new-moodle-themes/
Via File Manager
The 2nd way to upload is by the file manager of your hosted site (offered by sites like Bluehost and others). Here’s a quick example of how to do it using a host like Bluehost (similar to FTP, but browser based).