Hello , make changes to the template and does not like me , as I can restore the original template or manufactured. Cheers
If you have made changed to theme files, just delete the theme, and install it again.
If you want to load demo again, you can reset everything to state like it was when you first installed wordpress by using this plugin https://sr.wordpress.org/plugins/wordpress-database-reset/. After that you can import demo content again.
All the Best!