Step1. Update current distribution
apt update
apt upgrade -y
apt dist-upgrade -y
apt autoclean
apt autoremove -y
Step2. Change repo
apt install ubuntu-release-upgrader-core
cat /etc/update-manager/release-upgrades | grep lts
do-release-upgrade -d
Step3. Update System
apt update
apt upgrade -y
apt dist-upgrade -y
Step4. Clean up old repos
apt autoclean
apt autoremove -y
Step5. Reboot
reboot