User Reviews left on Schema enabled entries may all be seen in the WordPress admin area under Schema → Reviews.
From this admin page you may do many things to user reviews, this document will walk you through them.
Just like the way you edit WordPress pages or posts, you can edit user reviews exactly the same way. You can modify user reviews inputs by modifying post meta.
Add User Review Manually
You can also add user review manually just like creating a new page or post in WordPress.
Note that some post meta fields are automatically filled when submitting the review via the submit form.