Note: featured reviews is only available on the Standard plan.
First, navigate to Manage Reviews, then click on the review you would like to feature. In the single review view, you can click Mark as featured. You can also unmark as featured from the same location.
If you're using featured reviews, you'll likely want to think about it on a per product basis. Having 1-2 review(s) featured on each product. As such, you might want to get into this view:
• "Approved" folder
• Rating - "5"
• Product - find a single product
From there, you can more easily manage the reviews you're featuring for a given product.
If you're featuring reviews, make sure to enable Show featured reviews first in your On-site settings so that these reviews will be pinned to the top of your review displays. Check out this doc for more on this setting.