My advise would be that you should write a mail to the jeemain@nta.nic.in and tell them about your situation and ask them to chenge your number if there is not an option to change the number from the JEE official website .I hope that they solve your issue definitely.

Hello Deachin, this is not a big problem. what you need to do is go to your mobile operator office and request them to block your number and issue a new sim for that to you. It will be a easy process as the number would be registered in your name only. some documents will be required and the sim will be with you.

I hope you understand. All the best.