The details shared here apply to the most recent version of Flex. To keep your theme up to date, check out this helpful document.
In the Flex theme, you can enable a sidebar on collection, blog, blog post, and product pages. In version 1.1.4 (What theme version am I using?), a 'mobile sidebar position' has been added in. Now, you can now choose whether to display the sidebar above or below content on mobile for these pages.
To locate this setting, head into Customize (your Theme Editor) > select the Theme Settings Tab > click into Mobile > scroll down to Sidebar, where you'll find the 'Mobile sidebar position' setting. Here, you can choose between the 'above content' or 'below content' options.