Q: What should I do if my paper has been accepted but I have a modification to make?
What should I do if my paper has been accepted and is in the "In press" status, but I have a modification to make?
By “modification,” I am not sure whether you mean an “error” or a “change”, as both are different. While errors are present even in some published work, ideally, they should be spotted and corrected before going into publication. If you mean “change”, again, ideally, by this stage, the change should be minor.
The “In Press” status means that the manuscript is being prepared for publication. I am not sure if you have received the proofs of the manuscript by now. If not, you could make this modification when you receive the proofs and inform the editor of the same. If you have already received and submitted the proofed manuscript, and still wish to make this modification, you will need to inform the editor of this at the earliest. If the manuscript has gone into production, you won’t be able to do anything now. In the next issue, the journal may need to issue an erratum or corrigendum about the modification. In any case, please contact the editor at the earliest.