This article is for users who have made HTML/CSS edits to their site theme and need to revert back to a previous version. If you've never done this, then no need to read on!
If you ever need to switch to a previous version of your site theme - e.g. to roll back customizations or updates you've made, or to retrieve changes made to a pre-V2 Theme - then you can do this easily from your admin dashboard:
Head to Design your site > Theme library.
In your list of installed themes, find the one you'd like to switch to and select publish.
And that's it! You'll now see the theme you chose as the "My published theme" for your site.