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 


Then save this file

Then do a release upgrade

sudo do-release-upgrade -d

Once done, Then restart the machine and check again.