Custom actions kicktart commerce cart
You need to create a custom module and use hook_form_alter to modify the form.

That particular form id is views_form_commerce_cart_form_default, so in your custom module, you could have something like this:

function mymodule_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  //cart form
  if (strpos($form_id, 'views_form_commerce_cart_form_default') === 0) {
    //change the value
    $form['actions']['submit']['#value'] = "New Update cart value";
    $form['actions']['checkout']['#value'] = "New Check-out Value";

    //add additional callback functions on the checkout button
    $form['actions']['checkout']['#submit'][] = 'mymodule_callback';

    //replace callback functions on the checkout button
    $form['actions']['checkout']['#submit'] = array('mymodule_callback');
  }
}


Refer to the Form API Reference to see what else you can do.