For starters only your companies Administrator will have access to the inner settings to do this. To get to the section where you can alter and manage this you'll need to get into the BOL / Invoice Text under your settings. [as shown below]
Once in the BOL / Invoice Text Settings you'll open up to this window.
You'll want to scroll down until you see a section titled: Invoice Notes
This is where you'll be able to add in or edit the default notes for every Invoice. To edit the notes click on the pencil icon.