I'm not quite sure I understand what you mean. Do you want to compile the SCSS? If so you can use something like GULP or Webpack to compile it from SCSS to CSS (in case you already have a local work environment), or even online tools. It depends what you're trying to achieve.
Regarding about getting the latest Debut SCSS files, please bear in mind that CSS and SCSS are practically identical. SCSS makes it easier to develop with CSS adding features like nesting, mixing and variables, but if you're going to add custom features you don't necessarily need the latest Debut SCSS files, you need the latest Debut CSS, which you can find in their official download page.
Thanks for the reply Diego. I'm aware that I can just modify the existing CSS. I was hoping though the SASS might contain variables and other mixins which might have made the creation and customization of the css a bit easier. I heard that debut prior to v17.3 had the sass. Is there anywhere I can download older versions of the theme?
It's not possible to download previous versions of a theme. (source)
I don't think you'll find any sort of vital value in older Debut versions SCSS files that will have a considerable impact in development. However, with that said, I personally do have the Debut version 16.7.1 and I can happily send it to you.
Just forward me an email to firstname.lastname@example.org with the topic of "Shopify Community (old debut theme request)" and I'll attach a copy for you.