how to change "username" text located in registration form
The way to modify the user registration form is to add a hook_form_alter() function to your theme to change the form variables and add a theme hook to use a custom use-register.tpl.php file. There are numerous tutorials and responses here on stackexchange (for example) that can guide you on this process, but if all you're trying to change is one string, you should try the string overrides module, which allows you to easily replace anything passed through the t() function (ex. "username" --> "organization").