6 months into my pregnancy I told my boyfriend I had a one night stand with a coworker and the child could potentially not be his. He accepted it and still signed the paternity papers when she was born and raised her as his own for 2 months. He then decided to take an identigene test on his own and found out she was not his. I found the other guy took the same test with him and now its saying she's his biological father. How do I go after him now instead since my boyfriend signed paternity papers?

Where do you live?

The bio father needs a court ordered DNA test, which you should go to Family Court to get, if he won't.
Your boyfriend can go to court to get a DNA test ordered proving he isn't the father. Until all this is resolved, he is responsible for support, and the test he had done won't hold up in court.

now its saying she's his biological father

I assume you meant that she's the one night stand's daughter.

Any question on law needs to include your general locale as laws vary by area. The window during which a declared father can challenge paternity is defined by local law and can vary. Since your boyfriend singed an acknowledgment of paternity he remains the legal father until a court orders otherwise.

Local law also dictates who can challenge paternity. So the bottom line is you should get an attorney who can file a petition to change paternity and go for child support. You understand that the bio father can counter with a petition for custody and/or visitation.

I live in California. Me and my boyfriend are no longer together because of all this. It just upsets me how he accepted it and went on like nothing and even allowed me to give her is last name. What I want to do now is can I go after her biological father for support without having to deal with my ex since he signed declaration of paternity can I do that?

No, your ex is the legal father. Until that is changed, you have to deal with him. You have 2 years to rescind the acknowledgment of paternity:

California State Paternity Laws (http://www.dna-testing-paternity.com/paternity-laws/california-state-paternity-laws)

I would advise retaining an attorney to do this.

What happens after 2 yrs?

What happens after 2 yrs?

7638. (a) After the period for rescission under Section 7637 has expired, a signatory of an acknowledgment of paternity or denial of paternity may commence a proceeding to challenge the acknowledgment or denial only:
(1) On the basis of fraud, duress, or material mistake of fact; and
(2) Within two years after the acknowledgment or denial is filed with the Department of Child Support Services.

If the challenge is not filed within the 2 year window, then it will stand regardless of DNA evidence.

Ok sounds good with me:)

One more ? If my ex tries to take his name off her birth certificate or acknowledgement of paternity papers will I be notified or can he do it without me knowing?

No, he can't do that. He would have to go to court to challenge the acknowledge as indicated by the law I linked you to. That would require that you be notified of the court action.

Thank u everyone for all the helpful information I greatly appreciate it