Let’s assume you just downloaded a new theme which is in a zip file.

1. Extract the contents of your zip file and copy it under “Themes” folder.
2. Go to the admin section (www.yourdomain.com/admin).
3. Go to Configuration > Settings > General And Miscellaneous settings
4. Select new theme from “Desktop Store theme” drop-down and SAVE.
5. Now, go to public store. You should be able to see the new theme on your website.