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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

Since Unimus 2.0, 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 in-between versions.

If you are upgrading from v1 (any 1.x.x release), you will first need to upgrade to 2.0, and then you can update directly to the latest build in v2.