How do you handle with bad reviews about your product on the sites?

Remember that the customer who has given you a bad review is still a customer, and if you go to them directly with a mature and sincere attitude, they will likely be more receptive.

So, there are bad reviews and then there are bad reviews; I mean reviews given out of spite. If the review is about a genuine issue, then try to resolve it. But if it is out of spite or just to downgrade, then there is not much you can do about it. There are ways to handle this, and there are agencis that specialize in this, but it is not worth it for small players. After a point, you just get immune to it

You should always try to respond in a good way so that everyone else sees you're doing your best. If there's an issue, acknowledge it and mention you'll work on that.