简单:

• 找到其当前的上一个"元素
• 将其从树上删除
• 在找到的元素之前插入

类似这样的东西:

static void MoveElementUp(XElement element)
{
    // Walk backwards until we find an element - ignore text nodes
    XNode previousNode = element.PreviousNode;
    while (previousNode != null && !(previousNode is XElement))
    {
        previousNode = previousNode.PreviousNode;
    }
    if (previousNode == null)
    {
        throw new ArgumentException("Nowhere to move element to!");
    }
    element.Remove();
    previousNode.AddBeforeSelf(element);
}