using the StringBuilder methods Replace, Insert, Append, AppendFormat, and Remove:

   
 

using System;
using System.Text;

class UseSBApp {
    static void Main(string[] args) {
        StringBuilder sb = new StringBuilder("Pineapple");
        sb.Replace('e', 'X');
        sb.Insert(4, "Banana");
        sb.Append("Kiwi");
        sb.AppendFormat(", {0}:{1}", 123, 45.6789);
        sb.Remove(sb.Length - 3, 3);
        Console.WriteLine(sb);
    }
}