Typically, we do not delete reviews, because that would mean that you could never review this plugin ever again. You only get one chance, basically.
However, you can edit your own review at any time, including changing the rating you give the plugin. Simply go back to the reviews page for this plugin, scroll to the bottom of the screen, and edit your review and rating and save it. Done.
https://wordpress.org/support/view/plugin-reviews/wp-rss-aggregator