In jReviews 1.x it's a bit more complicated, in jReviews 2.x you can edit the /jreviews/views/themes/default/elements/detailed_ratings.thtml file where you'll find the output and just multiply by 5:
<td class="rating_label"><?php __t("Overall rating");?>:</td>
<td><?php echo $Rating->drawStars($review['Rating']['average_rating'], $this->Config->rating_scale, $this->Config->rating_graph, $reviewType);?></td>
<td class="rating_value"><?php echo 5*$Rating->round($review['Rating']['average_rating'],$this->Config->rating_scale);?></td>
I can't help you with the change for 1.x because it requires code changes instead of just a theme change.