How to save the data in session for new users and then fetch for modify and save into same session without using database connection

its very easy and I believe you can do this. Instead of putting all the information(name, age dob) separately in session, create an instance of type "User"(with name, age, dob properties) and set it in the session with a key.
To modify it simply retrieve it from session , then modify it and again set it in the session with the same key.