Can one Undo an Adoption when they are an adult?

Case in point:
Bio dad relinquishes rights to Child A at early age (2). Evil Step dad X adopts Child A and then later divorces mom and has not been in child’s life for 20 years… Bio dad comes back into the picture when Child A is an adult (30) with a family he (Child A) is reconnecting with bio dad and has a good relationship. He wants to have the adoption undone and have bio dad named as father on birth certificate.
How can this be done legally?

In many (all?) states, it’s not possible to undo an adoption*. It is, however, perfectly legal to do a new adoption; have bio-dad adopt his genetic son. That’s legal, and the consent of the person losing paternity is not usually needed in adult adoption.

But, again, adoption law varies by state, I am not anyone’s lawyer, much less yours.
*cite for Texas, for example

So one wouldn’t need to undo an adoption, they would just need to be readopted by bio parent…?

I believe it’s also possible for a child to legally sever all relationships with a parent. Presumedly, this would be the same as being “unadopted” if that were the original relationship.

Exactly. Check with a lawyer in your state, but I’ve never seen an exclusion for natural parents in an adoption law. And a person is not limited to one adoption - I know one unfortunate kid in my son’s class has been adopted three times, by different step-fathers.

Severing ties is easy, (relatively) painless (with an absent person), and free.

Unfortunately, it does not rectify the sitution of the legal document: the birth certificate. My limited experience with the adoption system (meaning the adopted son will be much better off contacting a lawyer in the state where the adoption took place), would suggest that the birth parent re-adopting the child would be the best solution.