To change the default invoice template please to go Settings > Templates and look for these:
invoice_print: used for printing the PDF invoice. Add your company name, contact details and logo here.
invoice_task_list: the task list used for outputting the pdf template
invoice_print_basic: a different basic invoice template, choose this from the ‘advanced’ area of the invoice page when generating a PDF
invoice_task_list_basic: the task list that goes with the basic invoice template.
credit_note_pdf: used for printing a PDF credit note (when an invoice has been converted into a credit note)
external_invoice: used when the customer clicks the “external” link sent to them in an invoice email
invoice_email_due: the email which is sent to the customer when the invoice is due (ie: the first time you sent an invoice to the customer)
invoice_email_overdue: the email which is sent to the customer when the invoice becomes overdue (manually sent through the interface)
invoice_email_paid: the email which is sent to the customer thanking them for invoice payment
invoice_payment_*: various templates used when displaying payment information to the customer
If you wish to edit the default template invoice_print you can rename it to invoice_print_mine so that you still have access to the default template.