Hello,
1) Have you made the icon font that includes that icon or just created a simple image?
If you created simple image then you can add the following code to child theme functions.php and add link to your image
function et_cart_summ() {
global $woocommerce;
?>
<a href="<?php echo wc_get_cart_url(); ?>" class="cart-summ" data-items-count="<?php echo WC()->cart->get_cart_contents_count(); ?>">
<div class="cart-bag">
<img src="link_to_your_image" alt="Cart" style="width:20px;">
<?php esc_html_e('Cart', 'legenda'); ?>
<?php echo wp_kses_data( sprintf( '<span class="badge-number">%1$u %2$s</span>', WC()->cart->get_cart_contents_count(), _nx( 'item for', 'items for', WC()->cart->get_cart_contents_count(), 'top cart items count text', 'legenda' ) ) );?>
<span class="price-summ cart-totals"><?php echo WC()->cart->get_cart_subtotal(); ?></span>
</div>
</a>
<?php
}
2) You provided screenshot of the FontAwesome 5.0 but Legenda uses the FontAwesome 4.7, so you can choose only 4.7 icons or additionally customize child theme to support FA 5.0.
Regards