Edit Global Default Email in Notifications 2016
As a Notifications administrator, I would like to be able to edit the global default email. so that users within my organization that want to build notifications use a more customized standard email template to base their notification email on.
Mike Zboray commented
It is not clear which version you are talking about but in 2016 R2, in PI System Explorer, go to Tools > Global Formats. There you can edit the global formats including the default format.