I'm assuming you have used DOM to get a Document with your XML data in it.
By "value" I assume yuu mean the text between the start end end tags, like the "xxx" in this example:
To do that, you need to 1) Find the <a> element. 2) Delete all of its existing children. 3) Create a new Text node with the new content you want. 4) Add add it as a child to the <a> element. 5) Do anything else you want to do to the Document. 6) Serialize it to save the XML file.
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