Moving backwards in a solo canoe is one of the more challenging things to do. You can't easily see where you're going, and you need to use strokes that you won't use on a regular basis. The most common stroke to use is the reverse J-stroke because the pry at the end of the stroke helps move you in a straight line. You can also use alternating reverse strokes and cross backstrokes to go straight backward, although this combination works best in the beginning when you're accelerating from a still position.
Moving backwards in a tandem canoe is challenging because it involves a full reversal. The bow paddler is now responsible for steering the canoe, which is best done with reverse J-stroke, while the stern paddler simply provides propulsion with reverse strokes. Paddling comfortably backwards in a tandem will take lots of practice, although it certainly isn't a necessary skill. Unless you're in a really tight spot, you're best to just pivot around and paddle forward.