Try:
string input = "2015-09-01,2015-09-03,2015-09-06,2015-09-10";
string output = input.Replace(",", "\",\"");
Except, you probably really want to do this:
string input = "2015-09-01,2015-09-03,2015-09-06,2015-09-10";
string output = "\"" + input.Replace(",", "\",\"") + "\"";
In order to balance up the double quotes:
"2015-09-01","2015-09-03","2015-09-06","2015-09-10"