Ah, do you mean that you don’t want the site to be responsive?
I would strongly recommend against doing that – it’s bad for users and it’ll negatively affect your site’s SEO in Google Mobile search.
Google explicitly recommends that all sites be mobile friendly.
The desktop version of a site might be difficult to view and use on a mobile device. The version that’s not mobile-friendly requires the user to pinch or zoom in order to read the content. Users find this a frustrating experience and are likely to abandon the site. Alternatively, the mobile-friendly version is readable and immediately usable – https://developers.google.com/search/mobile-sites/
If you still don’t want the site to be responsive, you’d have to write a considerable amount of CSS to override the media queries from the bottom of style.css – there is no simple option to make a site non-responsive.
You can place custom CSS in Appearance > Customize > Additional CSS.