Check Ubuntu version before upgrading.
lsb_release -a
First, we have to run an update
sudo apt update
Then run upgrade command
sudo apt upgrade
After that dist-upgrade
sudo apt dist-upgrade
Then update core manager
sudo apt install update-manager-core
Then edit below file
sudo vim /etc/update-manager/release-upgrades
At the end of the file set
Prompt-lts
Then save this file
Then do a release upgrade
sudo do-release-upgrade -d
Once done, Then restart the machine and check again.
