You should be able to change this with either of below two settings:
1. Application default templates:
Application Options->File->Default templates. This is to set the application default templates, but can be overridden by the Inventor project setting, see below #2.
2. Inventor project templates:
In current active Inventor project, you can set the templates folder from Folder Options->Templates. The Inventor project templates path will override the application default templates path if you change it.
Hope this helps.
Rocky Zhang Inventor API Manufacturing Solutions Autodesk, Inc.