Bitnami has documentation for administrative tasks, including upgrades. Unfortunately I have no idea which Linux distribution is used for Bitnami VMs. Suggest SSH’ing into the box and running cat /etc/os-release to figure out whether the package manager is apt (Debian/Ubuntu), yum (CentOS/RHEL), zypper (SuSE), etc. Then follow Upgrading from 13.12 to Latest - #2 by iwalker
Cheers,
Michael