Answer:
through naturalization.
More info:
Anyone born in the US is a citizen by birth. Other than this a person can acquire or derive citizenship.
A child born abroad to US citizen parent or parents is a citizen of the US as long as a few eligibility conditions are met by the parents. This birth can be recorded within 5 years at the Consulate abroad. The document issued at registration is proof of the child's citizenship. The child can also get a Certificate of Citizenship and a US passport as further proof.
When the parents naturalize as US citizens, any child they have below the age of 18 and having a green card is automatically granted citizenship. This child will have to get a Certificate of Citizenship and a US passport as proof of US citizenship.