When I edit the items on an order, are the totals and stock updated?

Yes. Open the order and click Edit. You can search for products, add or remove them, change Qty, and adjust the Shipping and Tax %. Then click Save changes.

When you save:

• The subtotal, tax and total are calculated again from the new items
• If stock was already reserved for this order, the old items go back into stock and the new items are taken out
• An "Order edited by …" entry is added to the Timeline, showing who made the change

If the customer already paid, check whether you need to collect or return the difference. That isn't done automatically.