To unvoid an order, open your order tab and either check the statuses so that only the Void orders appear:
Or if you have the order number, search for it with the Order ID: field
When you search for the voided orders, to the right of them, you will see a curly blue arrow:
This is the unvoid button. Clicking this takes you to a new page:
Here you can move the order to another status.