Recently I found myself needing to perform a downgrade of Home Assistant in Hassio which wasn’t immediately obvious. If you need to downgrade your Hassio install enable the ssh add-on (or the web based one) and enter the following command on the console:

curl -d '{"version": "0.57.1"}' http://hassio/homeassistant/update

The command will hang for bit while Hassio downloads the different versions and prepares to install it.