Doing it in CSS for all pages that use the "side nav" (what we call the left column) would be doable, though it'd likely require a fair few changes and decent knowledge of CSS, as you likely need to change the system to use flexbox and then reorder the columns via that.
If you want to do it via a small template change, you can change "sidenav" references to "sidebar" in the appropriate templates, though note that this will change the behavior on small devices (the sidenav collapses to a button which triggers a menu; the sidebar moves below the content).