Hello, 

You need to use the search or browse method. 

First of all you need to find the record id and after need to update. 

For eg. 

In Sales Order one boolean field and we need to mark active when invoice created. 

In that case first of all we need to search the record. 

I'm assuming that there is one field is Many2One and it's related to 'sale.order' and field name is sale_order_id.

def action_post(self):
        res =super().action_post()
        order_id = self.env['sale.order']search([('id', '=', self.sale_order_id.id)]) 
        order_id.valid = true
        return res

Thanks.

Suppose you have inherited a sale.order object and added a "Valid" field as below,

class SaleOrder(models.Model):   
    _inherit = 'sale.order'    
    valid = fields.Boolean("Valid")

Now you need to inherit the action_post() method (CONFIRM button) of the account.move object and add code to activate the checkbox of the sale order object.

class CreaditNote(models.Model):
    _inherit = 'account.move'

    def action_post(self):
        res =super().action_post()
        order_id = self.env['sale.order']search([('id', '=', 1)]) # Apply domain as per requirement.
        order_id.valid = true
        return res

In this case on click of confirm button on the credit note it will activate the checkbox on the sale order where id = 1