You can manually use the console as recommended by @aelsing, which works well enough.
But the more robust solution is to create a package which encapsulates your changes. See this thread for more info: Zammad Package Best Practices
To get you started, here is how you would change the branding:
- All settings are defined in db/seeds/settings.rb
- The brand name is called
product_name - You can get the current value with
Setting.get("product_name") - You can set it with
Setting.set("product_name", "New product name")
For creating users and such, just use the appropriate model method, for example: User.create_if_not_exists(..params..)