After reinstall emqx, we could not start emqx any more. I tries several times and find the access to solve this problem without losing external configs include emqx.conf and plugins’ configs

## stop emqx
sudo systemctl stop emqx 

## disable from systemd
sudo systemctl disable emqx

## remove from apt
sudo apt remove emqx

## delete old emqx files, without emqx.conf and plugins' configs
sudo rm /usr/bin/emqx
sudo rm /usr/bin/emqx_ctl
sudo rm -rf /var/lib/emqx
sudo rm -rf /usr/lib/emqx


## reinstall emqx
sudo dpkg -i emqx-ubuntu18.04-v3.2.2_amd64.deb

no matter you use apt install or dpkg -i to install emqx, you will occur such problem when need to reinstall emqx

if you use dpkg -i to install emqx, you should use dpkg -r emqx or dpkg -P emqx remove emqx before reinstall