then iterate over the indexes and with YourDTVar.Rows(idx + 1)(YourColNameOrIndex) = NewValue you can update the value
For Both mentioned approaches maybe you will extend a last row check (as there is not a LastRow + 1 index). We also can modify / doing some variations on the mentioned approaches as well
(From i in Enumerable.Range(0,YourDTVar.Rows.Count)
Where YourDTVar.Rows(i)("Col1").toString.Trim().Equals("YourSearchTerm")
Where Not i + 1 >= YourDTVar.Rows.Count
Select x = i + 1).toArray
With this variation we have already calculated the next row index and handled the last row check
For 1 column value update we would not recommend LINQ /RowArray reconstruction approaches. Also with the calculated indexes we had minimized the loop efforts.