It is possible to edit a standard Template. You'll first need to copy the template from the Standard Templates. Once you have done this, you will be able to edit it like any other Document Template.
Select Settings
Select Templates & Files and click Document Templates
Select Event Temple Standard Templates from the top of the page
Find the Document Template you want to use and Click the Preview 'Eye' Icon
If you wish to use this Document Template, click the Copy and Use button at the top of the preview
Make the desired changes.
When complete, click the purple Save icon at the top-right.