How to edit the particular column values in the dataset?

You can do like this directly .
Foreach(DataRow dr in dataSet1.Table[0])
{
dr["col1"]=//set some value

//or if you want to add something to exisitng values, then
dr["Col1"]=Convert.ToInt32(dr["Col1"].ToString())+5;
}

or if you want to assign to some variable.thats also you can do
Foreach(DataRow dr in dataSet1.Table[0])
{
int i=Convert.ToInt32(dr["Col1"].ToString());
dr["Col1"]=i*5;

}