You can add in rating_option
table but from it may get conflict with already exiting customer ratings and reviews.
fields are : option_id rating_id, code, value, position
If you want to install fresh magento script then in set up file.
magento\app\code\core\Mage\Rating\data\rating_setup
You will find for loop in it change it to your digit.
for ($i = 1; $i <= 5; $i ++) {
$optionData[] = array(
'rating_id' => $ratingId,
'code' => (string)$i,
'value' => $i,
'position' => $i
);
}
But this only works at the time of module installation.
You need to change this before set up magento.