Change datagrideview calculation using combobox index change

hello friends,
my problem is how can i change temprary value in datagrid view, when i combobox select value change then change all calculation of rows which is added in datagrid.I have 1 combobox where custmer name listed,
when i add value in datagrid, now i want change custmer name i'm salect custmer of other city then all caluculation change which is befor added.
i have one combobox_selectedIndexChanged event but i dont understand how to do.