How to edit datetime column in a dataset?

My dataset has column ‘ START_DATE’ and its in datetime format.

this is what i tried so far.

foreach (DataRow dR in codeDs.Tables[0].Rows)
{
    var def_startDate = Convert.ToDateTime(dR["START_DATE"].ToString());
    dR["START_DATE"] = def_startDate.ToString("dd/MM/yyyy");

    var def_endDate = Convert.ToDateTime(dR["END_DATE"].ToString());
    dR["END_DATE"] = def_endDate.ToString("dd/MM/yyyy");
}

However, this chunk of code is not updating the dataset.
Thanks in Advance!