In Word, in the File menu, point at Send To and then click PowerPoint.
In PowerPoint, in the File menu, point at Send To and then click Word.
But note, not all the content may be sent. From Word to PowerPoint only text
with level styles (Heading styles) will be transposed. From PowerPoint to
Word, only text in placeholders will be transposed in an outline fashion to
a Word outline. Or pictures of the slides will be transposed, with or
without the notes.
In other words, all the little extra goodies you may put into the slides,
like manual textboxes, markups, and such will have to be manually copied
into Word.
From Word to PowerPoint all text that has no level style applied will have
to be manually copied.