Krishna, if you have meant NEET application form, NTA will open its correction window from January 15th to January 31st and during this period, you can change your Category. Because this is a one time correction facility, make sure you make a list of all the corrections that you need to complete on your application form, edit the fields and then click on "Submit" after you are done with all the necessary fields. Details like name of Candidate, contact/ address details, category, PwD status, educational qualification, date of birth, etc will be treated as final. Any request or mail for changes in information after the closure of the correction period will not be considered by NTA under any circumstances. So it is advisable to edit the Category only if you have the relevant documents.