Fatal Error when installing plugin

Hello,

I'm upgrading from an older version

Fatal error: Cannot redeclare class Wiki in /var/www/vhosts/ca.lp.org/html/wp-content/plugins/wiki/wiki-include.php on line 11

I also get a message that says the /wiki/ directory already exists. Anybody know what can be done. Currently I have wiki light installed and I'm trying to install the full version.