If you have already paid the interest, the primary goal is to formally map that payment to the new demand order. File a reply in Form GST DRC-06 providing the proof of your past payment, and use Form GST DRC-03A on the GST portal to link your previous payment to the currently issued demand order. If you need further assistance, consider consulting with a tax professional who can review your specific filings to ensure the interest was paid in the correct cash ledger head.