Add upgrade instructions

website/site/content/docs/install/downgrading.md

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+title = "Downgrading"
+weight = 37
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+
+Downgrading is currently not supported!
+
+Note, it is not safe to install a previous version, because the
+database will not be compatible. Therefore, it is recommended to take
+a backup of the database before upgrading.
+
+Should something not work out as expected, you need to restore the
+backup and then go back to the previous version.
+
+## Docker-Compose
+
+The default `docker-compose.yml` file points to images using the
+`-LATEST` tag. You need to edit this file and replace `-LATEST` with
+the concrete version, like `-v0.20.0`.
+
+Then run the three steps as when upgrading:
+
+``` bash
+$ docker-compose down
+$ docker-compose pull
+$ docker-compose up --force-recreate --build -d
+```
+
+
+## ZIP / Deb Files
+
+Simply download a concrete version and re-install it using your
+package manager or unpack the zip file.
+
+
+## Nix
+
+When using the provided nix setup, the `currentPkg` always points to
+the latest version. But most other versions are also provided and can
+be chosen from:
+
+``` nix
+# …
+       docspell = callPackage (docspell.pkg docspell.cfg.v0_20_0) {};
+# …
+```