I cancelled an order by mistake. Can I change it back, and what gets restored?

Yes. Open the order in Orders → Manage Orders, pick the right status in the dropdown and click Update Status. The Edit button comes back once the order isn't Cancelled.

What is and isn't restored:

• Stock: cancelling released it. It's reserved again only when you set the order to Processing. Choosing Pending or Shipped doesn't take stock.
• Coupon, wallet, reward points and affiliate commission: these were reversed when you cancelled and they stay reversed. Wallet money and points went back to the customer, and the coupon use was freed.

So if the customer paid part of the order from their wallet or with points, collect that amount another way, or note it on the order with Add note.

Both status changes stay in the order's Timeline.