Can I update my research paper before the camera ready date?
I would say the common-sense rule is that minor updates are ok, but major updates - roughly, something that makes it into another paper, or would have required a new peer review - are to be avoided unless explicitly authorized.

Adding a few more sentences mentioning related work is fine, but if you think your changes are significant, describe them in a few (short) sentences and ask the editor/program committee chair.

Also: consult your co-authors before making changes and before requesting authorization for the changes!