Hi Janath

This kind of customisation is one of the trickiest parts of developing with Odoo, and poor approaches to this are one of the most common reasons I've seen apps break (especially app store apps, they love to override base code and not call super).

There are two main approaches:

Changing where the hook is: It's much better not to rewrite Odoo code, so trying another approach is always preferable. Is the code you're trying to edit actually being run in the _cart_update function, or is this function calling another function with some values that you'd like to change, or something else? If another function is being called, you can add a flag to the context before calling _cart_update, then make the change the other function that is being called if that flag exists. 

Overriding the original function: Rather than inheriting it, you can override the definition. You can do this with the following code:

from odoo.addons.website_sale.models.sale_order import SaleOrder as OriginalSaleOrder


def _cart_update(self, product_id=None, line_id=None, add_qty=0, set_qty=0, **kwargs):
    # Insert the original code with your changes here

OriginalSaleOrder._cart_update = _cart_update

This just replaces the website_sale level of _cart_update, so all other modules will still function as normal, no need to call any super.