I would like to change the template completely

I would like to change the template completely.

Where can I edit it? How do I access these files? Which ones are they?

I mean the default template
I don’t want to edit it from the admin panel because I can only add additional css code to it…

Thanks for help !

I think the following posts can help you with this.

It’s almost always better to use a theme than a plugin. Themes are much easier to install and maintain than plugins and are much less likely to disable your site in an upgrade.