how do you upgrade marketpress lite

how do you upgrade marketpress lite to marketpress full edition? I get an error when activating the full edition plug in:

Plugin could not be activated because it triggered a fatal error.

Fatal error: Cannot redeclare class MarketPress_Shopping_Cart in /homepages/26/d377220475/htdocs/classic cufflinks/wordpress-3.3.2/wordpress/wp-content/plugins/marketpress/marketpress.php on line 6698