Question - WPF datagrid add new row with default values, then edit?

I update the database after every cell edit.  Datagrid is bound to a datatable.  If a new row is added via the datagrid, I capture that event and put a value in a column in the new row in the datatable.  That creates a row in the database table, with data just for that column.  This is what I expect to happen.

Now I'd like to be able to edit other columns on the datagrid for that row and update that newly added row in the database table.

I'm not sure of the syntax.  How do I "tell" the datatable that now it's supposed to edit the row and not add?

Thanks.

Kris Hood