Q: What should we do if the first author was changed after acceptance?

Before we respond to your query, there is something that doesn’t seem clear. In the details of your question, you have said that you have submitted a revised manuscript. However, in the main question, you seem to suggest that your manuscript has been accepted. From what we can gather, presently, your revised manuscript needs to be re-reviewed, after which the editor will make their decision on the manuscript. It doesn’t seem to have been accepted just yet, unless you did mean that it’s been accepted. We wanted to highlight this, because if it’s been accepted, we can understand your concern in wanting to have it corrected as soon as possible. If not, there may not be much cause to worry.

Coming now to your actual query, this seems to be a mistake on the journal’s part, either by an individual or by the system. In which case, the onus is on them to ensure this is changed. However, you will of course need to write to them about this. The earlier, the better, to ensure there is no misunderstanding later (especially if your paper has been accepted). The editor may at the most ask your team/group of authors to validate that you are indeed the first author, but as this was established during the initial submission, this may not be something to sweat about. :) So, you may go ahead and write to them about this.

Now, in case your manuscript has been accepted, the next stage from your perspective is proofing. You may wish to prepare for it by going through this article: What is the minimum number of comments to be included while proofing my paper?

And in case the manuscript is still under (re-)review and awaiting a final decision, you may find this article interesting: Editorial decision-making: what are the possible outcomes for a manuscript?

Hope all that helps. All the best for the next stage for your manuscript!