Thank you for using Bitnami. Bitnami images already use Debian Stretch but you probably launched an old Bitnami image that used Debian Jessie.
In order to upgrade the operating system, you can either upgrade the operating system by your own or migrate the application to a new server with the latest versions of the application and all its components.
We recommend following the second approach as it updates all the components of the application (Apache, PHP, ...) to their latest versions fixing possible security vulnerabilities. We have a section in our documentation that explains how to migrate the information from one WordPress to a new one using the All in One Migration plugin.
This plugin is free if you use it in the single site solution but it's a paid plugin if you want to use it in the multi site one.
In case you decide to update the operating system by your own, please remember to create a backup of the instance before starting the process so you can recover it easily if something goes wrong
Happy to help!
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