Hello larcar
You do not need to access phpPgAdmin to update RosarioSIS. Simply overwrite the new files over the old ones.
phpPgAdmin on cPanel gives the owner error, indeed. cPanel only allows you to enter phpPgAdmin using a single user, which unfortunately is not the user who created the database tables, sequences, views, etc.
Maybe your hosting service can give you access to phpPgAdmin with the right user as specified in the config.inc.php
file.
Please let us know if you find a solution.