easy, redownload. reinstall... and following the installation steps.
Step 1: Edit the config.php.examplefile and rename it to config.php.
Step 2: Edit the .htaccess file in the root zenphoto folder if you have the mod_rewrite apache module, and want cruft-free URLs. Just change the one line indicated to make it work
Step 3: Change the permissions on the 'albums' and 'cache' folders to be writable by the server chmod 777 cache
Step 4: go to zen/setup.php