Naveen ;

Yes,you can change your category from EWS to OBC in your application. It is only applicable during the period of correction window. If you are speaking about NEET exam which is more likely, I'd like to inform you that the correction window will start on 15th of January and continue up until 31st of January. During that period only you need to visit the NEET website and change your category there. So, wait for the correction window to begin.

Hope this helps!