After searching a lot on web I found the solution
for (int i = 0; i < mycookie["RollNo"].ToString().Split(',').ToArray().Length; i++)
{
if (RollNo == mycookie["RollNo"].ToString().Split(',').ToArray()[i])
{
mycookie["RollNo"] = String.Join(",", mycookie["RollNo"].ToString().Split(',').ToArray().Where(val => val != RollNo));
mycookie["StudName"] = String.Join(",", mycookie["StudName"].ToString().Split(',').ToArray().Select((s, index) => new { s, index }).Where(x => x.index != i).Select(x => x.s));
}
}
Thanks to coaders...