Have you established that the graphs are, in fact, linked to an external Excel file? If they are, that's the only place the data can be edited. If they've just been inserted into Word as chart objects, double-clicking on them should open them for editing, in which case you should be able to access the data behind the charts and change the legend. Of course, if they've been inserted as images, that option won't be available (depending on the image format, you may be able to ungroup the elements and edit them individually).

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]