Vue Pinia how to update a property inside an object

I'm using pinia and I would like to know, how to update a property inside an object. I have an array of objects inside the state.cart, which are some products, which all have a property called quantity. This property can be changed, so I need to "update" the cart data.

This is what I've tried:

state: () => ({
   cart: []
}),

actions: {
  updateQuantityOfProduct(product, val) {
    const prod = this.cart.find((item) => item.id === product.id)
    prod.quantity = val
    this.$patch({
      cart: this.cart,
    })
  },
}

but this seems not to work, since the cart is not updated and/or when I refresh, the update/change has not been added (the quantity of the product is like before the change)

how can I solve this and what am I doing wrong?