How can I switch back to an older version of the theme?

On WordPress.org 

You can find and download all the previous versions of the Customizr theme in the WordPress.org platform.

For example, if you need to download v3.4.20, use this link : https://downloads.wordpress.org/theme/customizr.3.4.20.zip

You can change the version number (3.4.20 in the example) by the one you need. 

If you don't know which version you want, the version history is available here :https://wordpress.org/themes/customizr/