change value's key in dictionary
===

I have done as follows

      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];

I want to change the key when I move the cell in tableview.
I found value of key  could't be changed.

    - (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath
    {
        NSUInteger fromRow = [sourceIndexPath row];
        NSUInteger toRow = [destinationIndexPath row];
     
    }

what should I do...... in the above method.