Install an earlier version
Every published version is kept in the plugin's GitHub repository. That is useful to reproduce a behaviour on a specific version, or to roll back quickly when an update breaks something on your site.
Back up first
Rolling back does not undo data migrations that already ran. Have a backup before swapping the installed version — UpdraftPlus does it for free.
Once the .zip is downloaded, installing it is the same as in
Installation: upload the file under Plugins → Add plugin → Upload
plugin.
Going back from a 2.x to a 1.x
Version 2.0.0 rewrote the workflow builder and migrates old workflow content on update. Going
back to a 1.x after that migration can leave workflows unreadable to the older version. Restore
the backup instead of just swapping the .zip.