I have an example at work which I will post tomorrow when I go into the office
The method as I recall involves creating a node which is an xpath selection of the node you are looking for. Then changing it and saving the details.
I've read a couple of books on XML and it still makes my brain hurt too. I always get the impression when reading about it that the guy writing the article/book thinks xml is simple and sees no need to explain it as simply as possible.