Only guests can change their Moodle guest account password through Moodle.
To change the password through Moodle.
1. Login to your Moodle guest account.
2. Click "Change password" under Settings block on the left hand side.
3. You can change the password by entering "Current password", "New password" and "New password (again)". Please click "Save Changes" at the end of the page.
For HKU users, they use HKU Portal to login Moodle platform. HKU users can change their HKU Portal PIN by filling a form CF-111 at http://www.its.hku.hk/forms