In the end all you use is vanilla css and that’s what you upload. You write your css using sass and when you save it a corresponding css file is automatically created. Remember browsers don’t know anything about sass or less. All they understand is normal css. You just upload the css files as usual but keep the sass file as source and you just edit the sass file when you want to make changes and then upload the css file it creates for you.
I don’t use pre-processors myself but I managed to work my way through the sass tutorial on learnable in a day (including installation).