Q: Can I change the first author of my journal article after submission?

This is not an ideal situation. I am assuming that you are the submitting author, i.e., the author who submitted the paper on behalf of all the contributing authors. To begin with, you should never have submitted your paper to the journal without deciding upon the order of authors. And if the current first author did not make any significant contribution to the study, he/she should not have been attributed as the first author at all because that is unethical.  

Typically, journal editors do not encourage changes to authorship, particulary after a manuscript has been processed. When an authorship dispute arises after a paper has been submitted/accepted for publication, journals require all the authors involved to give consent for this change. So in your case, it would be advisable to write to the journal editor stating the reason for the change. If you are the author who submitted the paper, you should lead this communication. But know that the editor would want the current first author to sign off on this change because editors are not responsible for determining authorship, the authors themselves are. And consent/approval of all authors regarding the order in which they are listed is essential. So you may need to ensure that the researcher who is currently listed as the first author consents to this change. This may not be a pleasant situation - communicating with the journal editor and first author about this - but it is the most ethical one and one I would advice you to follow.

Check out this case study published by the Committe on Publication Ethics (COPE), which discusses a case where the order of authorhip of a paper changed after acceptance.