The general idea with SASS is you write a file named (for example)
Then you compile that file (using a pre-processor such as PrePos) into
the output file style.css.
You then upload this file to your web server.
You will never need to change the file style.css, any edits you make
will be to style.scss which is then compiled.
Initially, when you're converting from .css to .scss I'd make a backup
copy of your original CSS for reference, then, compile to SCSS and check
that the site looks the same, thus proving that your SASS is correct.