How to save XML node back into XML file with LINQ-to-XML?

Well, the error has nothing to do with saving, or even with replacement - it has to do with you trying to create an XElement without specifying the name. Why are you trying to use Select at all? My guess is you just want to use Single.

I think the problem is simply your use of the Select call in the statement assigning oldElementToOverwrite. You actually seem to want the Single extension method.

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