Other forms: backpedaling; backpedaled; backpedals; backpedalled
When you backpedal, you completely change your position or opinion. You might backpedal on your promise to buy your friend lunch when you see how expensive the restaurant is.
If a politician supports a certain position and then seems to reverse her opinion or significantly change it, she backpedals. And if you guarantee you'll make cupcakes for a bake sale, but then you run out of time, you might have to backpedal. A more literal meaning of backpedal is, just as it sounds, to pedal a bicycle backwards.