Commit changes from a datagrid

I am setting a cell in an autogenerated datagrid to the value from a message box input.

((TextBlock)DataGrid1.Columns[5].GetCellContent(DataGrid1.SelectedItem)).Text = inputMessageItem.Input;

Everything seems fine up to this point the value is displayed in the datagrid but the new values are not submitted back to the model.

It does not seem like the datagrid recognizes that the cell has been changed how can I force this?