How to override existing quantity already in cart

hi,

I want to override existing quantity already in cart for grouped products. I got this code from stackoverflow

Please help me to find the solution…..

add_action(‘woocommerce_add_to_cart_handler’, ‘update_product_in_cart’, 11, 2);

function update_product_in_cart($p, $q) {
global $woocommerce;
$cartItem = $woocommerce->cart->cart_contents;
$currentProductId = $q->id;
$wCart = $woocommerce->cart->get_cart();

// If cart already exists, and product exists, than remove product, and add the new product to it.
if ($wCart)
{
$cart_item_keys = array_keys($wCart);

foreach($cart_item_keys as $key)
{
foreach($cartItem as $item)
{
$productItemId = $item[‘product_id’];
if ($productItemId == $currentProductId)
{
// If you want to empty the entire cart…
// $woocommerce->cart->empty_cart();
// If you want to remove just the product from the cart…
$woocommerce->cart->set_quantity($key, 0);
}
}
}
}
// This adds the product to the cart…
return $q;
}