Once you file nothing can be changed. First you will need to wait and see if your return is Accepted or Rejected by the IRS.If the return is Rejected you can go back in and fix the mistake without amending. If the return is Accepted you will need to WAIT until you receive your refund and then Amend to add your dependent. 

Follow these steps to amend:

https://ttlc.intuit.com/replies/3288565