What to do about an important typo mistake in my submitted conference paper?

What you should do is to e-mail the organizers to tell them about it. Then, either they will let you replace the paper with a new version or they will not.

By the way, it happens quite often that papers contains some minor errors. Some reviewers will notice them while other will not notice them. It depends on how much time each reviewer spends on your paper and how attentive they are when reading your paper.

Contact the organizers immediately. For conferences with a deadline on a Friday, Saturday or Sunday, it's fair to assume that the final reviewer assignment won't take place until Monday.