ubuntu16.04 update 18.04 cannot get the new version

1. sudo apt update (更新软件源)

　　2. sudo apt upgrade (更新内核相关的包)

　　3. sudo apt dist-upgrade

　　4.  sudo apt-get autoremove

　　5. sudo apt install update-manager-core

　　6. sudo do-release-upgrade

using the above code, if you cannot get the new version;you should set the software&updates.