Hyperstage supports the UPDATE statement. For more information, see the PostgreSQL 9.2 Documentation.

UPDATE tbl_name    SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
    [WHERE where_condition]
    [ORDER BY ...]
    [LIMIT row_count]

AUTOCOMMIT should also be turned off during large UPDATE operations and the transaction explicitly committed as described in the INSERT statement section.

UPDATE can be used to maintain slowly changing dimensions, but if there are massive changes to the dimension, you might consider recreating the dimension with an ETL tool and simply dropping and reloading the dimension in the warehouse as this will improve performance.