how to change hte middle node in linked list C

// C# implementation to modify the  
// contents of the linked list 
using System; 
  
class GFG 
{ 
      
/* Linked list node */
public class Node 
{ 
    public int data; 
    public Node next; 
}; 
  
/* Function to insert a node at  
the beginning of the linked list */
static Node push(Node head_ref,  
                  int new_data) 
{ 
    /* allocate node */
    Node new_node = new Node(); 
      
    /* put in the data */
    new_node.data = new_data; 
          
    /* link the old list at the end  
    of the new node */
    new_node.next = head_ref;  
          
    /* move the head to point to the new node */
    head_ref = new_node; 
      
    return head_ref; 
}  
  
static Node front, back; 
  
/* Split the nodes of the given list  
into front and back halves, 
and return the two lists  
using the reference parameters. 
Uses the fast/slow pointer strategy. */
static void frontAndBackSplit( Node head) 
{ 
    Node slow, fast; 
      
    slow = head; 
    fast = head.next; 
      
    /* Advance 'fast' two nodes, and  
    advance 'slow' one node */
    while (fast != null) 
    { 
        fast = fast.next; 
        if (fast != null) 
        { 
            slow = slow.next; 
            fast = fast.next; 
        } 
    } 
      
    /* 'slow' is before the midpoint in the list,  
        so split it in two at that point. */
    front = head; 
    back = slow.next; 
    slow.next = null; 
} 
  
/* Function to reverse the linked list */
static Node reverseList(Node head_ref) 
{ 
    Node current, prev, next; 
    current = head_ref; 
    prev = null; 
    while (current != null) 
    { 
        next = current.next; 
        current.next = prev; 
        prev = current; 
        current = next; 
    }  
    head_ref = prev; 
    return head_ref; 
} 
  
// perform the required subtraction operation  
// on the 1st half of the linked list 
static void modifyTheContentsOf1stHalf() 
{ 
    Node front1 = front, back1 = back; 
      
    // traversing both the lists simultaneously 
    while (back1 != null) 
    { 
        // subtraction operation and node data 
        // modification 
        front1.data = front1.data - back1.data; 
          
        front1 = front1.next; 
        back1 = back1.next; 
    } 
} 
  
// function to concatenate the 2nd(back) list  
// at the end of the 1st(front) list and  
// returns the head of the new list 
static Node concatFrontAndBackList(Node front, 
                                   Node back) 
{ 
    Node head = front; 
      
    if(front == null) 
        return back; 
      
    while (front.next != null) 
        front = front.next;  
          
    front.next = back; 
      
    return head; 
} 
  
// function to modify the contents of 
// the linked list 
static Node modifyTheList(Node head) 
{ 
    // if list is empty or contains  
    // only single node 
    if (head == null || head.next == null) 
        return head; 
    front = null; back = null; 
      
    // split the list into two halves 
    // front and back lists 
    frontAndBackSplit(head); 
          
    // reverse the 2nd(back) list 
    back = reverseList(back); 
      
    // modify the contents of 1st half  
    modifyTheContentsOf1stHalf(); 
      
    // agains reverse the 2nd(back) list 
    back = reverseList(back); 
      
    // concatenating the 2nd list back to the  
    // end of the 1st list 
    head = concatFrontAndBackList(front, back); 
  
    // pointer to the modified list 
    return head; 
} 
  
// function to print the linked list 
static void printList( Node head) 
{ 
    if (head == null) 
        return; 
      
    while (head.next != null) 
    { 
        Console.Write(head.data + " -> "); 
        head = head.next; 
    } 
    Console.WriteLine(head.data ); 
} 
  
// Driver Code 
public static void Main() 
{ 
    Node head = null; 
      
    // creating the linked list 
    head = push(head, 10); 
    head = push(head, 7); 
    head = push(head, 12); 
    head = push(head, 8); 
    head = push(head, 9); 
    head = push(head, 2); 
      
    // modify the linked list 
    head = modifyTheList(head); 
      
    // print the modified linked list 
    Console.WriteLine( "Modified List:" ); 
    printList(head); 
} 
} 
  
// This code is contributed by PrinciRaj1992