Now, when you ask a horse/pony to canter, you should be asking them like this: Inside leg on the girth, slight pressure. Outside leg just a bit behind the girth, with harder and more constant pressure. This asks them to pick up the correct lead.
If your horse/pony does not pick up the correct lead, and it knows how to do a flying lead change (ask your trainer/friend if your pony knows, if you don't know, or just try this a few times), then all you should have to do is what I mentioned above: inside leg on the girth, outside leg behind the girth, and try and slow her down a bit with your hands and seat so she knows that you're asking her to change and not just speed up. Sometimes it's easier for the horse or pony to do a flying change when cantering over a pole on the ground, which you might try.
A flying lead change is much harder than a simple change, which is what I'd suggest you do. A simple change is when your horse/pony has picked up the incorrect lead at the canter, and to get it back, you simple go back to a sitting trot for just one or two strides, and then ask them to canter again with the above 'inside leg on girth/outside leg behind girth' method. Usually, especially with a trained horse or pony, they'll pick up the correct one after breaking to a trot.
Hope that wasn't too confusing!