To decline a feedback request navigate to the feedback request. You can read how to do that here.
You'll see the following view, to decline the request click 'Decline' as highlighted.
You'll then be prompted with the following:
If you want to proceed and decline the request click 'Yes, decline request' and if not simply click 'Cancel'.
If you do proceed with the decline, a decline notification will then be sent to the person who requested the feedback.