1. Pull down the Edit menu, choose Select All
2. Pull down the Format menu, choose Change Case, then whichever selection is most appropriate from there.
Select the whole document. Then go to Format > Change Case and select
Sentence Case.
It may not be perfect, since Word assumes that any word that follows a
period, question mark, or exclamation point must start a new sentence.
The cleanup shouldn't bee too bad, though.