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.