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With that letter, there should have been a "Request for Hearing" form. At this hearing you will have the opportunity to request the child support order be modified.
You may find this link helpful. There are no indications that you have to get a notarized Affidavit, but the Request for Hearing may state on the document itself that the signature must be notarized.
Hope this helps. Let me know if you have any further questions.