If you've used other Shopify themes prior to setting up your storefront with Flex, then you may be familiar with "theme styles" (previously known as presets). In other Shopify themes, theme styles are different looks that you can choose from within a single theme. Each theme style will have a default set of fonts and colors, which can help you set up your shop.
In the Flex theme, we currently have 12+ demo shops as theme styles.
Changing theme styles
Since the concept of "theme styles" in the Flex theme differs from other Shopify themes, there is no "change theme style" button with the theme editor.
Upload an alternate theme zip file from the 'unzip-me-first.zip' file
When you purchased the Flex theme, the theme downloads as a zip file called 'unzip-me-first.zip'. Once the file is unzipped, a folder called Welcome to Flex gets created.
There will be a main flex.zip file, as well as zip files named after the current demo shops (ex: flex-swagger.zip): 
If you want to start off with a different demo shop as your theme style, you can upload your demo shop zip of choice to Shopify. The theme will install with all of the sections, content, and settings as seen in that demo shop.