Update extra user data through Ninja Forms on WordPress

I have created a form using Ninja Forms, where users give some answers for some questions and create a score. I have calculated the scores in Ninja Forms, and I need to update that score as a user field. I have created the field using Profile Extra Fields plugin. I've seen the Custom Form Action and Hook Tag part, but I have no idea how to write the code and implement it. Can you guys help me?

If your intention to do it programatically here is one way of doing it (you can probably paste this in your functions.php, but preferably you should create custom plugin):

First register a new action:

add_filter( 'ninja_forms_register_actions', 'register_my_nf_action' );
function register_my_nf_action( $actions ) {
    $actions['my_action_name'] = new MyActionClass();
    return $actions;
}

Then create a class:

class MyActionClass extends NF_Abstracts_Action {
    protected $_name     = 'my_action_name';
    protected $_timing   = 'late';
    protected $_priority = '100';

    public function __construct() {
        parent::__construct();

        $this->_nicename = esc_html__( 'My Action', 'text-domain' );
    }

    public function save( $action_settings ) {
        // You don't need to implement anything here
    }

    public function process( $action_settings, $form_id, $data ) {
        // Check if user is logged in (WordPress)
        if ( is_user_logged_in() ) {
            $user_id = get_current_user_id();
            // some logic here
            update_user_meta( $user_id, 'meta_key', 'your_meta_value' );
        }

        return $data;
    }
}

Make sure to add custom action to your form and save the form before testing it.