As with any other system update, there is a chance that a newplugin or theme update may accidentally screw up your website.
This is a pain in the a......
WordPress doesnot allow us to revert to the previous version or does not has this feature build in.
But you can install this plugin "WP-Rollback" to get that functionality.

When you activate it, the plugin will add a link “Rollback” on any installed plugin.
This isreally something you need to have when you have some "exotic plugins"running...