The submission feature is available in the "Student portal" (which, for some reason, remains untranslated as "Estudante"). To get to this portal, you just click on its tab in the intranet page:
The feature is listed in the left menu, under "Submit" → "Projects":
The link leads to the project submission page, which lists the courses you're enrolled in and their assignments. For each assignment, you have a "View Submitted Files" link, and that's what you have to follow to submit your work:
The "View Submitted Files" link leads to a page where you can see the name, description and deadline of the assignment, download previously submitted files and submit a new file. So, to submit, follow the "Submit File" link:
This gives you a file upload form:
After submitting, you're sent back to the "Submitted Files" page, where you can now see the file you just submitted in the list. To download the file, click on its name.
That's all, the file you submitted is now in Fénix :-)