string s = "This is string";

s.RemoveAt(2);

string s = "This is string";
s = s.Remove(2, 1);

Console.WriteLine(Regex.Replace("This is string", @"(?<=^.{2}).", ""));

s = s.Remove(2, 1);

public static class MyStringExtensions
{
     public static string RemoveAt(this string s, int index)
     {
         return s.Remove(index, 1);
     }
}

string s = "This is string";
s = s.RemoveAt(2);