Xml?

I've just started trying to work with XML, and I'm a bit curious about something. Am I correct in assuming that XML files are static in nature? I'm not sure I'm making myself clear, so please allow me to try and explain with an example from the book I'm looking through.

One of the examples in the book, dealing with seperate XML and XSL files, deals with a bunch of couches, as though the website's being set up for a shop. The basic structure of the code I'm wondering about is as follows:

<sofa>
<style code="s123" color="red">Bali</style>
<size>3-seater</size>
<price>449.99</price>
</sofa>

Basically, what I'm asking is this- Is there any way to change the data in the file (s123, red, Bali, etc...) without having to actually open the file and recreate it? I'm more used to PHP than to XML, so I really have no clue where to go. My question probably has an obvious solution to it, but if it does, I'm too new to XML design to see it. Any assistance would be greatly appreciated.

Thanks in advance,
-EnderX