I had an affair and the child is not my husbands so how an I go buy getting dna testing

What is your question? It is hard to understand.

In most places of the US, if you are legally married, your husband is the legal father of your child.

ANY question on law needs to include your general locale as laws vary by area.

As noted your husband will be the legal father. If you want to challenge his paternity, this involves going to court. Who can challenge and when depends on local laws.

What is your question? It is hard to understand.

I think she was trying to ask "how do I go about getting DNA testing?" And if, by this question, she means to ask how she can get a court-adjudicated ruling on paternity based upon a DNA test, I don't know that she can. Her husband or the biological father could perhaps petition the court for a determination.