While developing an application and writing some migration code (using zope.app.generations), I had the need to reset the generation number recorded in the database for my application to a version lower than the current generation number (because my generation code didn't run properly and I didn't want to create bogus generation files). To solve this issue, in a pdb prompt I had to run:

(Pdb) db = self.request.publication.db
(Pdb) conn = db.open()
(Pdb) conn.root()['zope.app.generations']['myapp.generations'] = 0
(Pdb) import transaction
(Pdb) transaction.commit()
(Pdb) c

Not much to it, and this info can be easily obtained by reading the zope.app.generations source code.