Step 1: Change directory into your path that installed the ghost blog. For example, mine is:
cd /var/www
Step 2: Download the latest verion using wget
Note: You could find the current verion in the official website: https://ghost.org/download/
Step 3: Remove the old core code
rm -rf ghost/core
Step 4: Unzip the downloaded file
unzip -uo ghost-0.6.4.zip -d ghost
Step 5: Change ownership and permission
*chown -R ghost:ghost ghost/**
Step 6: Install new dependencies
cd /var/www/ghost
npm install
Step 7: Restart ghost
pm2 restart ghost
Note: If you are not using pm2, you may run the below command instead:
service ghost start