Changing Select list to radio button and edit "Select a Product"

You'd have to alter the Add to Cart form in a custom module; there's no place to do this in the UI.

That code will work for the product select list shown in conjunction with product attributes. You'll need a different variable for a standalone product list, $form['product_id'].

<?php
function MYMODULE_form_alter(&$form, $form_state, $form_id) {
  if (strpos($form_id, 'commerce_cart_add_to_cart_form_') === 0 && isset($form['product_id'])) {
    $form['product_id']['#type'] = "radios";    
  }
}
?>

Just remember to sanitize the #options values when you change it to radios.

Nope, iterating over the #options array and passing the values through check_plain() to ensure HTML / JS is escaped properly.

I am developing a module that you may be interested.

http://drupal.org/sandbox/facine/1687512

a greeting

These are the code comments for the select list the code above alters:

          // Note that this element by default is a select list, so its #options
          // are not sanitized here. Sanitization will occur in a check_plain() in
          // the function form_select_options(). If you alter this element to
          // another #type, such as 'radios', you are also responsible for looping
          // over its #options array and sanitizing the values.

Thank you for your feedback Ryan! Here's the updated code where options are sanitized:

<?php
function MYMODULE_form_alter(&$form, $form_state, $form_id) {
  if (strpos($form_id, 'commerce_cart_add_to_cart_form_') === 0 && isset($form['product_id'])) {
    // Sanitization of select options will occur in a check_plain() in
    // the function form_select_options(). We change this element to
    // another #type, 'radios', and hence we are also responsible for looping
    // over its #options array and sanitizing the values.
    foreach ($form['product_id']['#options'] as $key => $value) {
      $form['product_id']['#options'][$key] = check_plain($value);
    }
    // Change element to #type radios.
    $form['product_id']['#type'] = "radios";    
  }
}
?>

@jessicakoh: Sanitize = convert to plain text -> remove html / js. Without this, html / js included in product titles will render in product options, posing a security threat.