You can find the important stuff relevant to youi in
/opt/zammad/app/assets/javascripts/ the application.js is Zammads javascript file.
After changing the file, you need to tell Zammad the changes by:
systemctl stop zammad
zammad run rails assets:precompile
systemctl start zammad
Changes should take affect then.
If needed, you can remove your current javascript compiles to ensure zammad really writes them: rm /opt/zammad/public/assets/*.js*