Using string.ToUpper on substring

Have an assignment to allow a user to input a word in C# and then display that word with the first and third characters changed to uppercase. Code follows:

namespace Capitalizer
{
    class Program
    {
        static void Main(string[] args)
        {
            string text = Console.ReadLine();
            char[] delimiterChars = { ' ' };
            string[] words = text.Split(delimiterChars);
            string Upper = text.ToUpper();
            Console.WriteLine(Upper);
            Console.ReadKey();
        }
    }
}

This of course generates the entire word in uppercase, which is not what I want. I can't seem to make text.ToUpper(0,2) work, and even then that'd capitalize the first three letters. Only solution I can think of now that would make the word appear on one line (and I don't know if it works) is to move the capitalized letters and lowercase letters into a character array and try to get that to print all values in a modified order.