Take the date as a string in the current text format: "01-jan-2019" and use TryParseExact to convert it to a DateTime value.
You can then use ToString to output it in any format you want:
Formatting a DateTime for display - format string description[
^]
string input = "01-jan-2019";
DateTime dt;
if (DateTime.TryParseExact(input, "dd-MMM-yyyy", CultureInfo.InvariantCulture, DateTimeStyles.None, out dt))
{
Console.WriteLine(dt.ToString("dd-MM-yyyy"));
}