How do I upgrade the theme? Will I lose all my changes?

To upgrade the theme:

Download the latest version from envato (installable Wordpress file)

In admin dashboard: 

remove the old theme and install the new version

using FTP remove the theme folder from /wp-content/themes/ and then copy the new version

You will not lose the changes if you added content, uploaded media or changed theme/page options.

if you haven't modified the theme files you should be fine.