Find it under "Appearance->Themes" and "Appearance->Premium Themes"
Under "Appearance" is where you can switch the overall look of your site, by choosing a theme. You can also switch themes at any time.
"Appearance->Themes": Themes that you have access to.
"Appearance->Premium Themes": Themes that are specific to paid subscription plans.*
*Note: If you are not subscribed to a plan, you might see themes that you can not activate under "Premium Themes". You can use the preview link to see what your site would look like with the potential new theme.