If you've not used PGAdmin before you might want to
backup your poker data before making any changes there as making database changes outside of PokerTracker 4 isn't supported. Close PokerTracker 4 and then you can access PGAdmin by clicking your Windows Start button then typing:
pgadmin
and then clicking on it when it appears. Double click on 'PostgreSQL x.y' (where x.y is your PostgreSQL version) and if your prompted for a password the default is 'dbpass' (without the quotes). Next double click on 'Databases -> YOUR_DATABASE_NAME -> Schemas -> Public -> Tables'. Right-click on the 'tourney_summary' table, choose 'View Data -> View All Rows', find the relevant tournament, edit the value in the 'amt_buyin' column and save the change (File -> Save'). If you have trouble finding the tournament you can sort by the 'amt_buyin' column using 'Tools -> Sort/Filter'.