The below process helps to make voided transactions to Un-voided.
1. Edit the Sales Order.
2. Enter a Quantity for the Items on the Sales Order. (In voided sales order quantity will become zero)
Since the Rate was not erased when the Sales Order was voided, it will automatically compute the Amount based on the Quantity entered.
3. Remove “VOID” message on Memo field.
4. Save the Sales Order.
Once you save the Sales Order the status will go back to Pending Fulfillment.