How to export a OneNote template?

Do you mean a Page Template or a templated notebook?

Page Templates cannot be saved separately as such. They exist in a special OneNote file at:

 %appdata%\Microsoft\Templates\My Templates.one

A notebook can easily be copied as it is a folder/file collection. Simply find the location on your drive. Options\Save & Backup shows you where the files are. You can also export the notebook to a package file.