Can't upgrade vue cli due to repeating "Please run npm install -g @vue/cli-upgrade and try again."

I've tried to upgrade vue cli, and noticed that I need to install @vue/cli-upgrade.

ueda@klingsor:~/ml$ sudo vue upgrade

  Command vue upgrade requires a global addon to be installed.
  Please run npm install -g @vue/cli-upgrade and try again.

So, I've installed @vue/cli-upgrade and try vue upgrade again, but the message is repeating.

ueda@klingsor:~/ml$ sudo npm install -g @vue/cli-upgrade
+ @vue/cli-upgrade@3.12.1
updated 1 package in 7.273s
ueda@klingsor:~/ml$ sudo vue upgrade

  Command vue upgrade requires a global addon to be installed.
  Please run npm install -g @vue/cli-upgrade and try again.

Also, the version of vue cli has still remained as 3.4.1

ueda@klingsor:~/ml$ vue --version
3.4.1

How can I upgrade my vue-cli to 4? Thank you for your suggestion!

My environment is ubunto 14.04, and npm is version 6.13.7

ueda@klingsor:~/ml$ cat /etc/os-release 
NAME="Ubuntu"
VERSION="14.04.2 LTS, Trusty Tahr"

ueda@klingsor:~/ml$ npm --version
6.13.7```

Solution

I fixed it by running:

sudo npm install -g @vue/cli