How to change a value of element or node in XML file

Question

Hi all, I have question that I have a xml file, I add it to my Project C# Win Form, Now I want to change a value of 1 node or element, so what I need to do? Please give a guide or link or example project to me reference, thank very much.
Here is demo file xml and picture demo.
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1. <?xml version="1.0" encoding="utf-8"?>
2. <data>
3. <book>
4. <bookID>1111</bookID>
5. <bookName>English</bookName>
6. <bookPrice>2$</bookPrice>
7. </book>
8. <book>
9. <bookID>222</bookID>
10. <bookName>USA</bookName>
11. <bookPrice>3$</bookPrice>
12. </book>
13. <book>
14. <bookID>3333</bookID>
15. <bookName>Singapore</bookName>
16. <bookPrice>4$</bookPrice>
17. </book>
18. </data>

http://i1055.photobucket.com/albums/s505/vn_photo/222.jpg