This is very helpful posted on Madeira Life Facebook page.
WITHDRAWAL AGREEMENT BIOMETRIC CARDS
[Applies only to those British citizens who arrived before 31st December 2020, have a Certificado de Residência for EU citizens (CRUE), and are in possession of the QR code issued when you registered for the WA biometric card on the SEF Brexit page.]
Here are the instructions for scheduling an appointment:
Go to the page [https://brexit.sef.pt/en/].
(https://brexit.sef.pt/en/?fbclid=IwAR2FxvyLGyYarAi2LOu38y4P0GI3b0rB0AleZwJbMGNdvuAVq92mt-bXbrM)
Log in. It should take you to your page, with all your details.
Click on the yellow button near the top, the one that looks like a calculator but is in fact Schedule an Appointment.
Tick the box next to your name. It will say “No scheduling”.
Click on the yellow calendar button again.
Select Loja do Cidadão da Madeira from the drop-down.
Click on one of the dates in white on the calendar.
Pick a time (if you’re offered a choice).
Click on the yellow computer disk symbol.
Read the warning and if everything is OK, click on the computer disk symbol again to confirm the appointment.