using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var case1 = "this is ood.";

        var case2 = "this is even.";

        Console.WriteLine(MyUpper(case1));

        Console.WriteLine(MyUpper(case2));

    }

    public static string MyUpper(string txt){

        var len = txt.Length;

        int upperIdx;

        if(len%2==0)

            upperIdx = txt.IndexOf(txt.Reverse().FirstOrDefault(char.IsLetter));

        else

            upperIdx = txt.IndexOf(txt.FirstOrDefault(char.IsLetter));

        var chars = txt.ToArray();

        chars[upperIdx] = char.ToUpper(chars[upperIdx]);

        return new string(chars);

    }

}

​

​