If you are looking for old versions of a plugin in the WordPress repository (like WooCommerce), you can go to the plugin’s page on WordPress.org/plugins, then look in the right column under the plugin’s Tags for the link to “Advanced View”. Click on that, then scroll all the way down and select from the dropdown under Previous Versions to download an earlier version. This is that screen for WooCommerce: https://wordpress.org/plugins/woocommerce/advanced/
After downloading the older version of the plugin, go to your WordPress control panel > Plugins screen and click “Add New”, the click the “Upload Plugin” button. Select the older version’s file, upload it, click “Install” and then “Activate”. (If you get a message at this point that the plugin cannot be installed because the directory already exists, you may have to deactivate the current plugin and uninstall/delete it before uploading the older version again. Make sure to make a backup of your site before you do this).
As far as determining what version you were using previously, I’m not sure how you’d determine that after updating. You may have to experiment.