You can cancel your "sent" friend requests whenever you want, provided the users haven’t already accepted them. Navigate to the friends section and find the "View sent requests" option. The other person doesn’t receive a notification about the cancellation anyway. To cancel (actually deny) "received" pending friends requests, the process is the same as above, except you won't click on the "View sent requents" option. The list of recieved ones should already be there.