Just like any standard software, you will have access to free updates for your theme during the life of your theme’s version number.
Your install files in your Aarade admin area are always the latest version. To update to the latest version of your theme, apply the installation process detailed in the support article How do I edit my theme's styles and options? This article includes instructions to help preserve any additional custom style coding you may have added at your Big Cartel admin.
To check whether your installed theme is the latest version, you can compare the details in the Theme Information section of your CSS file with the information provided in the Themes area of your Aarcade admin area (click the More link for your theme for complete version history).