Previously, I have updated the firmware several times, so when I saw there's a new update, I clicked the "Update" button without too much thoughts. Yes, I got trouble this time. I got a black screen while the system is not dead. I can log into the system by pressing Alt+Ctrl+F2, no gui, but I can do something to fix it. It is good enough. I searched on my cellphone to try find something. And the following article helped me.
In my case, I guess that the root cause could be the nvidia driver, so I followed the [Reinstall NVIDIA Driver] section in that article and it did save my life.
# remove current driver
sudo apt purge ~nnvidia
sudo apt autoremove
sudo apt clean
# reinstall it
sudo apt update
sudo apt full-upgrade
sudo apt install system76-driver-nvidia
In case someone else got trapped into the same situation, I wrote my solution here and hope it helps.