First of all, to change the date format, you need a value DateTime. You cannot format the date of a string. Use DateTime.ParseExact to extract your date value from a formatted date string:
DateTime dateValue =
DateTime.ParseExact(stringDateValue, "dd/MM/yyyy h:mm:ss tt",
CultureInfo.InvariantCulture);
DateTime.ToString(format), :
resultStringDateValue = dateValue.ToString("MMM dd, yyyy");