I signed the certificate the day after birth in the hospital and it disappeared and was replaced by another man with his name
1 attorney answer
If you are asking what you need to do to have the court determine the child's parentage, you likely need to file a petition with the court asking the court to determine the child's parentage.
The specific procedures depend on the specific facts of your case. Your post is missing a lot of specific facts.
You need to act quickly in case there are statutes of limitations for your case. If there are statutes of limitations, the main one likely is 4 years after the birth of the child.
You should promptly review the specific facts with your attorney to find out your legal options.