You could just edit the css file but its better never to touch the bootstrap file and just use your own custom file to do the over-rides.This leaves the system intact if updates are forthcoming etc.
If you want to set up or change variables and grid designs to suit your own purpose and preferences.
Assuming you are developing locally then all you need to upload is the css. Browsers only understand css, they do not understand scss. When you work with a pre-processor the code you is used is complied into actual css at the end and that’s the only css needed for the page to work.