The following is far easier to understand and use than the Outlaw's tip (IMHO).
text = Regex.Replace(text, "screen_fadetimeout=\"[^\"\']+\"", "screen_fatetimeout="99");
It has the following advantages:
1. It is one line of code
2. It is easier to read
3. It is faster (note: XmlElement uses Regex in it's implementation) and further speed can be gained by using a compiled Regex instance.
4. No need to worry about null references (what if the desired element doesn't exist?)
5. It is light compared to XmlElement.Parse()
6. If you need to know if the item was found, you can use the MatchEvaluator overload and an anonymous delegate -- still one line of code.