Found a solution from the HSC 2021 Facebook by HyAnh Nguyen:
UPDATE ON USYD ENROLMENT
You can only proceed these steps after you accepted your offer at USYD and received the "Enrolment is now open" email.
TLDR: enrolment for units of study and changes to personal details etc... occurs in the page that you previously went to to accept your offer.
1. Log in to your Sydney Student account with the 9 digit ID and password you changed to. If you have trouble logging in, use this link to change your password You can only proceed these steps after you accepted your offer at USYD and received the
2. You should see various tabs that used to have "My applications" there, such as "My details", "My studies"
3. Update your personal details, pick your majors/minors and select your units of study.
4. I think the unikey is supposed to be for your timetable but I haven't finished enrolling yet. Do let me know what happens after step 3.