Before you update, check your /opt to be sure you don't already have an observium_old directory or any tar files from previous updates. If you have an existing observium_old your backup will probably go into observium_old/observium. Then you just have to go into another directory to retrieve your config.php, etc.
If you have old tar files, the new one that downloads may have a ".1" appended on the end of it, so when you do the update you'll just be extracting the old one and won't get updated.
So see if you can find your lost config.php at /opt/observium_old/observium/config.php.