How can you lose 5 kg in 5 days?

In a healthy, realistic way, you can't.

In unhealthy, unrealistic ways, you could have surgery, maybe get really, really sick and lose that weight through fluid loss. Or camp out in a sauna and sweat it out. But then you'd just put it right back on the moment you start drinking. And it'd give you a whopping headache, maybe even hallucinations.