Changing the Organize Series Listing Pages

The template for the Series taxonomy is your-theme/taxonomy-series.php. Generally speaking, taxonomies use the taxonomy-{taxonomy-name}.php template which is outlined in the Template Hierarchy article in the Theme Development handbook. The diagram in that article is particularly helpful with determining what template WordPress is loading.

I double checked how the plugin registered the taxonomy by searching the plugin’s code for register_taxonomy. The plugin used the name series for the Series taxonomy, which is pretty straight forward. Sometimes the taxonomy name will not match the human readable name or a prefix may be added to the name, so it’s a good idea to double check this to make sure that the custom template is named properly.