How do I change our company details on our invoices?

To change details (e.g. address) for outgoing documents, go to the Document Zone, choose the Settings Tab, and the Formats Option.

Click General Details on the left and update as required.
Click Local Details on the left and update as required.

Click on Invoice Setup on the left.  It is split into two sections - the top for editing general settings, and the bottom for editing anything invoice specific.

Then you can click 'Preview Invoice' to verify the invoice is correct, and then repeat for the Statement setup on the left.