Did you try?
I think, direct change to it should have worked!
dataSet1.Tables[0].Rows[1].ItemArray[2] = "Test";
Also try:
ds.Tables[0].Rows[1]["Thisfield1"] = "Test";
ds.Tables[0].Rows[1][2] = "Test";
Found this for you to read:
How to: Edit Rows in a DataTable[
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