datagrids in C#

hi, in the datagrid, i have added different controls and populated the fields from an xml file. Now, i want to edit certain fields and it should be such that the datatable is updated abd the changes are reflected in the xml file. The updated records should then be visible in the cell where it was changed. But i am not finding any way to update a particular cell's value. Can any one help me? would really appreciate you help namrata