To edit system templates, you first need to unsync the email template. This means that if any changes are made to the 'synced' templates, the one you edited will not be affected by the changes, because it is no longer synced to our system templates.
To do this, you simply need to click to the right side of the content, in between the body of the email and the menu on the right.
This will give you the option to unsync the template and allow you to edit.
