Opening the configuration screen depends on the application and sometimes operating system your are using.
As a shortcut you can press Ctrl+, on Windows and Linux, or Cmd+, on macOS.
In the menu, click on Tools > Options.
Or press Ctrl+,.
In the menu, click on Joplin > Preferences
Or press Cmd+,.
Tap on the burger icon ≡ in the top left corner, and select Configuration.
Type :help config for information on how to set config values and for the complete list of available options.