How do I print an invoice for an order, and can I change its design?

Open the order and click Print invoice at the top. The invoice opens in a new window with the print dialog ready.

To change the design, go to Settings → Store Settings → Invoice Templates:

1. Click Preview on any template to see it with sample data. Click Set active on the one you want. The active template is the one Print invoice uses.
2. To make your own, click New template, give it a Template name and edit the Template HTML. Click the token chips to insert order data, for example {{store_logo}}, {{bill_to_address}}, {{items_table}} and {{grand_total}}. A live preview sits next to the editor.
3. Click Save template, then set it active.

Built-in templates can't be deleted, and you can't delete the active one. The PDF invoice attached to the customer's confirmation email uses a fixed standard layout, not your template.