Hi

You have not mentioned the name of the exam regarding which your question is but in general you cannot change your mobile number after your rank card or your result is been released

This is because,

To make any kind of correction one needs to use correction window and generally correction window is available only before the examination is conducted and  once correction window gets closed nothing can be done and no corrections can be made whether its mobile number or any other details.

I hope that you find this helpful

Always feel free to ask your doubts we are always here to help you and guide you

Thank you