You can change your last swipe selection by tapping the "Undo" icon. You cannot undo the selection if it resulted in a Match. 

Members can eventually turn up in your match pool again, even if you accidentally swiped left, or missed the opportunity to undo your left-swipe. 

Members that you left-swiped - that is, said "no" to - may still be able to see your profile in their own match pool or in the Explore feature.