how can i get selected value instead of selected text and assign that value to that cell where combo box appear
If you need the value and even the underlying key use EditingControlShowing event, cast DataGridViewComboBoxEditingControl.SelectedItem to the proper type then access the value. Lets say we are dealing with a DataTable then to get the underlying key int colorId = ((DataRowView)(((DataGridViewComboBoxEditingControl)sender).SelectedItem)).Row.Field<int>("ColorId");
If you don't need the value ASAP then cast the BindingSource.Current to (keeping with above) to a DataRow (first we cast to a DataRowView) then access the field.
If not dealing with a DataTable then cast to the class type and go from there.
Now with that said, the majority of developers simply have no clue on how to a) properly setup a DataGridViewComboBox b) know the events to use. Once those two items are clear it's simple to work with changing data.