How to update master details data and stock for purchase

Question

Hi
I need urgent help regarding the following.
I have a master detail relationship form. I need to give my client facility to modify / delete the row from the details table. I am able to display the rows in data gird view of the detail part. I can show the row in the respective textbox and combo box to update the required things. But when i try to update the things i am getting confused about how to update the stock from the stock table of the perticular item. My confusion is when should i update the stock table and stock. At the place where i update the whole purchase bill. I need some code for that. I am using ADODB for connection. Please help me this is very urgent.
Million Thanks in advance.