How can i update an element in collection instead of the reference

var productToUpdate = this.ProductSearchResults.FirstOrDefault(p => p.ID == product.ID);

if (productUpdate != null)
{
    productUpdate.Property = product.Property;
    ...continue for other properties
}

IN actual fact all you are doing is changing the reference to the local variable toUpdate to point at the passed-in argument product.

Lets take a step backwards, when you do:

var toUpdate = productToUpdate.First<ProductInfo>();

you have a reference to an item from your collection (ProductSearchResults). You can now happily update its properties, ala:

toUpdate.ProductName = product.ProductName;
toUpdate.Price = product.Price;
//etc..

however, you cannot update the itemn in the collection to point to a different/new item in the way you were attempting to. You could remove that item from the collection, and add your new one if that is indeed what you require:

public void UpdateProductInfo(ProductInfo product)
    {
        var productToUpdate = this.ProductSearchResults.Where(p => p.ID == product.ID);

        if (productUpdate.Count() > 0)
        {
            var toUpdate = productToUpdate.First<ProductInfo>();

            this.ProductSearchResults.Remove(toUpdate);
            this.ProductSearchResults.Add(product);
        }
    }

Hope that helps.