To allow users to vote product reviews as helpful, you must first enable the option Vote reviews as helpful that you find in Settings > User permissions.
To vote, they can simply click on Helpful on the review.
After voting, the toggle will change color and a message will show to let the user know their vote has been registered. To cancel the vote, they only need to click on Helpful again.
Moreover, users can also view all the most helpful reviews if you have enabled the dedicated tab in the review box.
