Ubuntu normally blocks direct root logins and leaves the root password unset. If you need to sign in as root—for example, on a system you control and where the regular account is no longer wanted—start by giving root a password.
Set the root password
Run the following and enter the new password twice when prompted:
sudo passwd root
Allow root over SSH
Open the SSH daemon configuration:
sudo vi /etc/ssh/sshd_config
Find PermitRootLogin and change its value to:
PermitRootLogin yes
Restart SSH so the new setting takes effect:
sudo systemctl restart sshd
Log back in as root and confirm that root access works before you remove the regular account. Testing first matters: if root login fails after you have deleted the other user, you can lock yourself out.
Remove a regular user
To delete the user together with their home directory, use:
userdel -r username
If you only want to remove the account and leave the user's files in place, run:
userdel username
To list accounts that can log in with a shell, check for bash entries:
grep bash /etc/passwd
For a full view of every account on the system, including built-in service users:
cat /etc/passwd
Once root login is confirmed and the regular account is gone, the system can be managed through root as intended.