Add value to meta_value in user_meta after order is placed

This function updates user_meta with meta_first and meta_second values. meta_first is textbox with number and meta_second is selectbox. Now every product will have different values for meta_first and meta_second.

// Update user_meta after woocommerce order is placed. 
add_action('woocommerce_thankyou' , 'wp_user_meta_update');

    wp_user_meta_update($order_id){
        # Get an instance of WC_Order object
            $order = wc_get_order( $order_id ); //get the order id
           $items = $order->get_items();       // get the item
            $user_id = $order->get_user_id(); //get the user id
            $current_user = wp_get_current_user();
            $user_id = $current_user->ID;

            foreach ( $items as $item ) {
                $product_name = $item->get_name();
                $product_id = $item->get_product_id();
                $product_variation_id = $item->get_variation_id();
               $get_meta_first= get_post_meta( $product_id, 'meta_first', true);
               $get_meta_second= get_post_meta( $product_id, 'meta_second', true);
               update_user_meta($user_id,  'meta_first' , $meta_first);
               update_user_meta($user_id, 'meta_second', $meta_second);  
            }
}

When any user places an order for a product let's say product one with the value of meta_first 10 the user_meta updates with 10. What I want to achieve is if the user places an order of product two after with the value of meta_first 20 the user_meta should update with 10+20.

You should move update_post_meta out of foreach block.

For example,

$get_meta_first=0;$get_meta_second=0;
foreach ( $items as $item ) {
  $product_name = $item->get_name();
  $product_id = $item->get_product_id();
  $product_variation_id = $item->get_variation_id();
  $get_meta_first= $get_meta_first + get_post_meta( $product_id, 'meta_first', true);
  $get_meta_second= $get_meta_second + get_post_meta( $product_id, 'meta_second', true); 
 }
 update_user_meta($user_id,  'meta_first' , $get_meta_first);
 update_user_meta($user_id, 'meta_second', $get_meta_second);