Simran, NTA, the conducting body for JEE MAINS allows the change from one category to another, say from General category to EWS as in your case, by filling the application form for April JEE as a fresh candidate. So, if you want to change your category in April JEE, you have to register as a fresh or a "new" candidate by entering all the asked details from the starting. The registration process for April session is easier for the already registered candidates who don't want to change the credentials entered for Jan JEE but if you have to change the credentials or details, you have to register yourself as a fresh candidate by going to the application portal and clicking on the link "fresh applicantions only" and register by entering all the details asked and enter your category as EWS. If you do so and register yourself as a new candidate, then you can give the April Mains as an EWS candidate and NTA would recognise you as a "new" candidate.