参考文档：https://www.hostwinds.cn/tutorials/how-to-upgrade-from-ubuntu-18-04-lts-bionic-beaver-to-ubuntu-20-04-lts-focal-fossa

#18.04 update 20.04 ，bionic 改成 focal 代号名称
sed -i 's/bionic/focal/g' /etc/apt/sources.list   
sed -i 's/bionic/focal/g' /etc/apt/sources.list.d/*.lis

sudo apt clean && sudo apt update  && apt --purge autoremove && sudo rm -r /etc/apparmor.d/cache

sudo apt install apparmor && apt upgrade -y   &&  sudo do-release-upgrade &&sudo apt dist-upgrade

lsb_release -a 
cat /proc/version
reboot