You can access the Feedback tool from your You page by clicking the big blue "Give Feedback" button at the bottom of the screen. From there, you can choose who to give Feedback about (this can be multiple people), write your message, choose from the regular four visibility settings (Public, Private, Private + Manager, or Manager only), and attach any relevant company values.
You can also view any public Feedback that you or your colleagues have received from the mobile app. Feedback about you will be located at the bottom of your You page. Feedback about others can be accessed from their profiles, which you can find in the company directory.