if a fly breaks her legs. how can we fix her little legs so she can fly again?

Unfortunately that seems like a lost cause. I hate to see helpless bugs like that die. We simply don't have the tools for operations on an animal that small. Especially an insect. Their hard exoskeletons make surgeries of any kind difficult. And if its legs are broken and not its wings I see no reason why it can't fly. It might have trouble landing. Also flies only live about 30 days anyways, so it wouldn't have much time to heal or live long after being operated on. It's as if you're asking to reattach a stinger to a bee.

I had an incident where I caught a dragonfly and 2 of it's wings fell off. They're incredibly sad to see try to crawl because their legs are insufficient for locomotion. I knew it would sit for hours before dying from starvation or something else so I retired it early. If you have an incapacitated fly I advise the same. You could try to prolong life as long as you can by putting it with rotting food to feed it. But its life would be useless in its eyes because it wouldn't be able to breed. That's what its 30 day lifespan is for.

Forget it man, they're broken

Just kill it.

Use a nail gun