Update DataRow from DataTable after select with Linq Query

Code:

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                    var results = from myRow in dt.AsEnumerable()
                                  where myRow.Field<string>("ProductID") == PMTDProductID && myRow.Field<string>("CustNo") == APMFCustNo
                                  select myRow;
                    int resultcnt = results.Count();

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I am using the above code to find if I have an existing DataRow in my DataTable that matches the productid and the customer number. If the resultcnt is 0 then I am going to add a DataRow for the customer and the product. How do I update the datarow if the resultcnt = 1? Where can I find the information on this? I have been searching all morning.