A paper with copied material We recently submitted a paper to a conference. However, I just noticed that one of the collaborators have copied three paragraphs from another paper of mine, though that part is not our main contribution.

He claimed that he has written that part, and he showed his work, but he has forgotten to apply the changes to the final version. I was wondering if I should withdraw the paper to avoid getting flagged as plagiarism or I should wait until the review results are out and then make changes to the paper?
First, deal with your collaborator, then notify the conference asap. If they figure it out and you then are like "oh, yes, sorry, I know, I will change it." you are in a bad place. Not only did you waste the reviewers time with a version you knew was wrong/has to be corrected, your late excuse might also not seem honest.

If, however, you inform them yourself, apologize for the inconvenience and the mistake and give the corrected version, they might well forgive you.