Your best bet is to just restore your last backup to a new non-dropbox location and upgrade. Then delete everything on Dropbox and move the fresh database back into dropbox in a new folder and delete the old one (to make sure there aren't any incompatibilities).
However, if you aren't running a backup (and if you are using this method, you should be doing it daily) your only option is to log onto the Dropbox website and manually go through each folder and restore files to their previous version. I stopped using Dropbox to sync PB after upgrading to 6.0. If you use PB daily and have two computers, at this point the WebBrain sync feature is worth the $7 ish a month just for that feature. It doesn't require you to go through the Dropbox workflow (making sure PB is closed on one machine and DB is synchronized before opening another) and is supported by folks at TheBrain, so if your brain gets corrupted you have the recourse of PersonalBrain support. Good luck!
Java SE 6