how do i modify the fields in “ADDITIONAL INFORMATION” in contacts
You cannot actually modify the fields in the “additional Information” of a contact record. Nor would you want to as most of them are used within the application itself for a specific purpose. So a date field must remain a date field with a specific format. You can however alter the labels shown on the contact record to be more meaningful to your application if needed. Check out his help article on how to do that.