I have finished my paper for a computer science conference, and I submitted it, but after the deadline a discover the missing of a figure, what if I updated my paper on the Easychair conference organization platform?
If the conference uses the Easychair system for organizing the reviewing process for conferences, then you can check if you can still upload an updated version. This often works, as many conference organizers choose to manually disable the submission of updated versions after the deadline has passed. So if Easychair allows you to still upload an updated version, you can use this functionality. This is because in this case, the assignment of papers to reviewers hasn't been triggered, so nobody should have had a close look at your paper already.
When you cannot upload updated version anymore, the best course of action is to send a new version to the PC chair, asking her/him to update the paper on Easychair manually. Accompany the new version with a list of changes and an apology. If this change list really short and lists easy to recognize problems with the paper (such as a missing figure), the PC chair is likely to forward your updated paper to all reviewers, so that they can do the review on the updated version. They may choose not to, though, because your updated version is late. But if a reviewer hasn't even started looking into your paper (and printing it), they will normally not mind.