Updating Posterno via FTP
This process is very, very similar to installing via FTP. The only real difference is that the posterno folder will already exist.
First, visit the following page to download the most recent version of Posterno: https://posterno.com/download
Once you have the zip file, open it (typically by double clicking it) and inside you'll file a folder called posterno.
This is where things differ from the install. Now you want to take the CONTENTS of the posterno folder, and put them inside the posterno folder that exists on the server.
Now go to the Plugins page in your WordPress admin area, find Posterno and deactivate and immediately reactivate it. This allows its install hooks to fire and check for database updates etc.
Then you're finished.