title: change value's key in dictionary
body: <p>I have done as follows</p>
  
  <pre><code>  NSDictionary *dic=[[NSDictionary alloc] initWithObjectsAndKeys:a1,@"0",a2,@"1",a3,@"2",a4,@"3", nil];
    NSDictionary *userbtn=[[NSDictionary alloc] initWithObjectsAndKeys:dic,@"button", nil];
    [[NSUserDefaults standardUserDefaults] registerDefaults:userbtn];
  </code></pre>
  
  <p>I want to change the key when I move the cell in tableview.
  I found value of key  could't be changed.</p>
  
  <pre><code>- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath
  {
      NSUInteger fromRow = [sourceIndexPath row];
      NSUInteger toRow = [destinationIndexPath row];
  
  }
  </code></pre>
  
  <p>what should I do...... in the above method.</p>