How do I change the order of user reviews so that oldest reviews are on top

JReviews 2.3 and newer

You can set the ordering in JReviews Configuration, tab Reviews:

JReviews 2.2

Edit \com_jreviews\jreviews\models\reviews.php

default:
	$order = '`Review.created` DESC';
	break;

change to:

default:
	$order = '`Review.created` ASC';
	break;