If you've followed the recommended way to install Postleaf, updating will be easy. Follow these steps to get the latest version.
First, login to your server and navigate to the directory Postleaf is installed in. If you've followed the DigitalOcean tutorial, the command will look like this:
To update to the latest version, run the following commands:
pm2 restart app
Again, this assumes that you've installed Postleaf using the recommended approach and are using pm2 to manage it. If you're using another process manager such as forever, you'll need to restart the app through that instead.
Updating a Theme
To update a theme you've added manually, simply update the appropriate theme files and restart the app.
To update a theme you've added via Git, navigate to the appropriate theme folder and run
git pull and restart the app.
In either case, when Postleaf is running in production, a restart is necessary to clear cached templates.
This update process will change once Postleaf is out of beta. Until then, this method will update you to the latest code in the master branch which I try really hard to keep stable.