Unless your database is small (fewer than 200-300k hands for example) it would be a good idea to back it up first - click "Start -> All Programs -> PostgreSQL 8.x -> Start Service" then follow the steps here (the "backup" steps only):
Using PGAdmin to backupThen for the export/import you're trying to do, you want to 1) export the affected hands, 2) purge those sessions from the database then 3) import the hands you've exported.
1. Export the hands to some temporary location like "C:/exported_hands" to make them easy to find for importing:
http://www.pokertracker.com/forums/viewtopic.php?f=45&t=175092. Purge the affected sessions:
http://www.pokertracker.com/forums/viewtopic.php?f=45&t=175513. Click "Import -> Manual Import -> Import From Directory" and in the window that pops up navigate to "C:/exported_hands".
Mike