Whenever possible, avoid modifying Bootstrap’s core files. For Sass, that means creating your own stylesheet that imports Bootstrap so you can modify and extend it. Assuming you’re using a package manager like npm, you’ll have a file structure that looks like this

bootstrap-project-file-structure.text

your-project/
├── scss
│   ├── variables.scss
│   └── custom.scss
└── node_modules/
    └── bootstrap
        ├── js
        └── scss