Prerequisite: You are a teacher, or as a parent / student the corresponding chat must be set to "Open".
For the file you want, click on the three dots to the right of the file name. As a parent / student, you can only delete files that you uploaded yourself.
You now have several options. By selecting "Delete" you can remove the file. To prevent accidental deletion of files, you must confirm once again that you really want to delete the file before permanently deleting it.