Your browser loads css file and sourcemap which has links to .scss file which gets compiled into css.
In your theme there should be a gulp or webpack script which is used to compile scss to css file.
If you are not familiar with scss->css compilation or you can’t find package.json and webpack.config.js or gulpfile.js files inside your theme’s directory you can either look for compiled css file which gets actually loaded and it’s usually minified to make edits. You can just override styles with yours in wordpress admin panel Appearance -> Customize -> Additional CSS.
If you’ll find package.json in your theme’s directory, you can run npm install command in console to install dependencies and inside package.json there can be a scripts section with build commands which you can use like npm run-script build to build css from scss files.
Again what I’ve explained is just general approach without knowing neither what theme you are using nor if this theme has those build tools used.
Important thing -> after you’ll make changes to css (or scss and rebuild css) and upload it to server don’t forget to CMD+R Ctrl+F5 to make sure your browser will load fresh updated css.
If you use git push to deploy to live site -> make sure you’ve added and committed all theme’s files, including css which is built from scss, so they got loaded to live site when you push those changes.