According to Cordell

Customizing phoenix generation templates

When using a framework’s code generators, its often useful to be able to tweak the code that is generated each time. In particular, the templates that are used to generate the HTML often need a custom structure to match your styling or fronted CSS framework.

In phoenix, this is done by adding new template files to your app at priv/templates/. For the HTML templates specifically, it would be at priv/templates/phoenix.gen.html.

As a starting point, you can copy the ‘base’ tempaltes from the phoenix github. Note that any template not present in this directory will default to original template.

Source