The nav cart’s display options can be changed via the theme customizer however you can also programmatically alter the display via the themedd_edd_cart_option filter.

/**
 * Change the display of the nav cart.
 *
 * @param string $cart_option The cart option to display.
 *
 * @return string $cart_option The cart option to display.
 */
function themedd_snippet_edd_cart_option( $cart_option ) {

	/**
	 * Show just the cart total.
	 * Possible options: item_quantity | cart_total | all | none
	 */
	$cart_option = 'cart_total';

	// Return the cart option.
	return $cart_option;

}
add_filter( 'themedd_edd_cart_option', 'themedd_snippet_edd_cart_option', 10, 1 );

PHP should be added to your child theme's functions.php file.