The Information Bulletin says that Candidates who belong to Minority community for example Muslim, Christian, Sikh, Buddhist, Jain and Parsi can select Minority as Yes in the application form. And there is a self declaration form in the Information bulletin in Appendix XIII. You need to download it, fill it up and sign it. And then that will act as a proof of the fact that you belong to Minority community. It's on page 85 at: https://ntaneet.nic.in/ntaneet/ShowPdf.aspx?Type=1F7D662EC0F41DE2013706728E830CBF545FD969&ID=D435A6CDD786300DFF204EE7C2EF942D3E9034E2

You can change your minority status in the correction window. NTA has opened the correction window on January 15th and will be open until January 31st. Login into the official website and make the necessary corrections.