The missing step from many online articles appears to be the switch to ghost-mgr.

After logging into a terminal session;

# Update package lists
sudo apt-get update

# Update installed packages
sudo apt-get upgrade

*if there is an error (after "apt-get update") downloading from Yarn, run this command:

curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add

https://github.com/yarnpkg/yarn/issues/7866#issue-558663837

sudo -i -u ghost-mgr
For DO droplets, elevate to ghost-mgr
cd /var/www/ghost
ghost backup
sudo npm i -g ghost-cli@latest
 ghost update