Hi,
You can find the .SchDot file in public documents, this document is editable.
Take the copy of this document, edit it as per your requirement and save it.
Then open new schematic which is in .SchDoc format, then go to Design/project templates/choose file
Now your .SchDoc file will be updated as in the .SchDot format.
And if you required your edited template as to be your default template whenever you open schematic document, set the path in preferences
Go to preferences under system select new document defaults there select schematic and browse for your edited template and add it.