Help with inserting a node/data to an existing xml file?

I am trying to store this data - this file will be updated periodically and the only parts that I need to add to will be under the <maintenance /> node. That is, <performed /> will be added periodically. I can create this file the first time okay, but on updates the entire file is written again, that is, all the nodes are rewritten.

I have a form that the user fills out and when a save button is clicked, only the <performed /> node and its attributes should be added to this file.

How is this done? - I've just about pulled the remaining strands of hair from my balding brow. If anyone could point me in the right direction, please. Thanks in advance.

Code:

---

<?xml version="1.0" encoding="utf-8"?>
<automake xmlns="http://tempuri.org/cavalier1.xsd">
    <owner>Your Name</owner>
    <optional>OptionalData</optional>
    <originalmileage>23</originalmileage>
    <auto year="2002" make="Ford" model="Mustang" color="Black" />
    <maintenance>
        <performed date=" " task=" " mileage=" " extradata=" " />
        <performed date=" " task=" " mileage=" " extradata=" " />
    </maintenance>
</automake>

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