What to do if I found out the preprint paper missed a citation of others' work? I recently found out my preprint paper (to be published in a journal) that missed a citation of others' work, the paper is just published online in IEEE Xplore? The sentences are rephrased, however, I forgot to give credit notice within the texts. I feel very badly about it. I know this is a plagiarism. I do not want to find the excuse for myself. I want to remedy it before I cannot. What should I do it now? 

The paper I should cite is in the reference list of the bibliography. I did not cite it within the texts.
Contact the journal as soon as possible with the correction, and see whether they can change it before the paper is officially published. 

When they had real (and not electronic) typesetting, you had to try to change the text as little as possible. That's still a good idea, although it's less important now.

If they can change it before it's published, that's ideal. If they can't, you can decide what steps to take after they tell you it's too late.