Hi Abdelwahed,

Are you talking about updating a value in an on_change? Please add some more information/code?
An example of an on_change that changes values:

 #This method will be called when either the field CostPrice or the field ShippingCost changes in the view.
    def on_change_price(self,cr,user,ids,CostPrice,ShippingCost,context=None):
	#Calculate the total
	total = CostPrice + ShippingCost
        res = {
            'value': {
		#This sets the total price on the field standard_price.
                'standard_price': total
	      }
	}
	#Return the values to update it in the view.
	return res