You should look at the code in the update cart function in the eCart include file. It has code that updates from a form, but someone with experience writing php would be able to pretty easily see how it was done.
The cart is stored as an object and the items in an array. So updating the column is just a matter of updating the cart object and the corresponding item in the item array. The update cart function should give you insight on how to reference that object and array.