You can change the metadata (author names, affiliations, abstract etc) at any time using the metadata form. See How do I edit/correct/enter article metadata before publication?
You can make minor changes to the manuscript in step 2 of the copyediting process, see What are the steps during copyediting?
The final chance to make minor changes is during proofreading of the PDF galleys, see What should the author do when proofreading the final galleys? Note that we will not allow major changes at this stage, this should be done during copyediting (see What if my proofreading corrections are rejected?).
If authors discover errors after publication we will need to publish a corrigendum, see We published a paper in JMIR and have a correction. What is the process of publishing a corrigendum? Additional charges may occur.