Disestablishing Paternity: Where Do I Go and What is the Cost to Rescind Affidavit of Paternity

My question involves paternity law for the State of: New Hampshire,
I am on the birth certificate of my ex girlfriends baby. I was told I was her father and later was told I wasn't. I did my own paternity test to find out the truth and it said there was no way I could be the biological father. My ex took me to court to get medical even though she said I was not the father and they told me the next step was to fill out a petition to rescind the affidavit of paternity so I went to the family court where they told me and they had me fill out the wrong form and when I went back they didn't know what to tell me and didn't even know if that was the right court. I was wondering if you could tell me what court I need to go to and how much it would cost. If it's a high cost is there any way around it because I am out of work right now and actively looking but don't want all my money go to medical for a child that isn't mine when I do start working
Thank you in advance

Disestablishing Paternity: Where Do I Go and What is the Cost to Rescind Affidavit of Paternity