Apparently, there are multiple ways??? I changed our companies default message this way (online via Google Chrome):

Click the gear icon (top right)

Click on 'Custom form styles'

BAM! There ya go. There should be an option to edit the default invoice message for ALL outgoing invoices!

Quickbooks/Intuit should REALLY make this more clear to others. There are multiple links to people asking this exact question and most of them had no luck... We pay good money for these services so we deserve better help and options.