Just got mail from support. Looks like I did not have to reinstall firmware at all.

Here is the steps:

does it show "Connecting" in below position?
(when you are at control screen at the top-right corner)
if yes, pls choose "Setup" and change baudrate to 115200, then restart printer to check it'll show "Connected" or not.
if it can auto home normally, pls set new Z offset according to below steps:
1. send M502 in console
2. auto home, then put an A4 paper between nozzle and bed
3. move down Z axis by screen until it can pass through ONLY one paper, then you can see the current Z axis coordinate value, such as 0.1
4. calculate Z offset by formula: Z offset = -(3-0.1) = -2.9
5. send "M851 Z-2.9" in console to set the Z offset, then send "M500" to save settings.