Quick answer: Open the Assignment and follow these instructions…

Full answer
Follow the instructions below to download student submission and marker comments from a Moodle Assignment dropbox.

1. Click on the dropbox link.
2. In the “Grading summary” page, click on “View all submissions”.
3. To download all student submissions, in the “Grading action” dropdown list, select “Download all submissions”.
4. To download specific submissions
   1. Select the student(s) in the list.
   2. Scroll down to the bottom of the list.
   3. In the “With selected” dropdown list, select “Download selected submissions”.
5. To download marker comments, in the “Grading action” dropdown list, select “Download grading worksheet”.