If you are using a self-hosted Ghost, you may have sometimes to upgrade it to the new release.
To do that, you have at first to download the latest Ghost release.
For that create a new folder next your current Ghost folder and download Ghost :

mkdir newghostfolder && cd newghostfolder
wget https://ghost.org/zip/ghost-latest.zip
unzip ghost-latest.zip

You can make a copy of your current ghost folder and you will have to remove the index.js, all the .md and .json and the core folder. But do not delete the config.js file.
Then you will have to stop the current Ghost instance by using :

forever stop index.js

Copy the new files over your current folder, and install the new dependencies with :

npm install --production

After than you can restart your instance

NODE_ENV=production forever start index.js