It depends on how you scanned it.
If you used OCR so that it is now editable text, then you need to select
area with the black background and use Format Borders and Shading, Shading
tab and remove the black background. If the text is formatted to Auto, then
it should revert itself to black text as soon as the background goes white.
If it doesn't, then select all the text and use Format, Font to select Auto
Text colour.
If you scanned the document into Word as a graphic object, then you cannot
do the above. The best solution is to copy it into a graphics editing suite
(such as the free to download Paint.NET or IrfanView) and use the Invert
Colours tool.
