1. send them Fax and registred letter with your file# let them know about the changes in your application. Pay $100 fees or they may ask you to fill up new update info in new form, wait for the instructions.
2. Send the Birth Cerificate copy, if you have B.C copy from hospital it will not be accepted ,you have to apply with Hospital letter to County office and ask for 4-5 copies of Birth Certificate ( you have to pay extra for each copy) becase you may need them for Canada immigration, for US passport and If you wish to remain in US then for the child to attend School.
3. once you have Birth Certificate copy apply for the Child US passport immidiatly.