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If you are using HSQL (the embedded file-based DB), simply make a backup of the database directory.
Windows: "C:\ProgramData\Unimus\hsql"
Linux: "/etc/unimus/hsql"
Update sequence
Unimus updates are designed to bring your Unimus instance up to date from any previous version. You do NOT need to upgrade version-by-version.
What this mean in practice is that you can upgrade directly from for example 2.1 to 2.6, and you do NOT need to deploy the inbetween versions.