Once you open a design, simply click download source file from the bottom toolbar.

https://cdn.elev.io/file/uploads/YCivSR8Fm6JjATHHNiKVQnFkJ0ucXiR5__THw6ZhwLk/qJDzWLzwv2vPLyEcxVE-RDMQ-s5MnkOLZ8fnbUV3nWA/DownloadSource-KKc.png

You can turn off downloads or lock it until the design is finalized from the project settings.

Here is an article on locking downloads.