C# 中的xml问题？

Question

   C#题目：要求在代码里面修改xml内的某个节点的Innertext值？
            急急急！！！各位大哥大姐，请帮帮忙啊！
                        十分感谢！！！

Answer 1

               string state=  "";  //这里是node2.PreviousSibling.InnerText值        
                                  case "item":
                                  if (node2.InnerText == node.Text)
                                   {
                                       int num = i * j-1 ;//获得选中项的lable索引，从而将状态值改变
                                            foreach (Char ch in state)
                                       {
                                          if (num == x * y-1)
                                           {
                                        state = state.Remove(num, 1);
                                        state = state.Insert(num, "1");
                                 node2.PreviousSibling.Value=state;                                                      
                                             }
                                         }                                        
                                    }
                                     break;
  问题在这里：：：：：我要将改变后的state值保存到node2.PreviousSibling.InnerText中？？？？
                       我用它的Value属性试过啦，可是保存不了？？？？？？急急急，请各位帮帮忙啊!!!