Update upgrade.md (#1467)

docs/docs/content/upgrade.md

@@ -17,4 +17,40 @@ Some versions may require changes to the database. These changes or database "mi
 - Head to your dashboard, and select your Listmonk project.
 - Select the GitHub deployment service.
 - In the Deployment tab, head to the latest deployment, click on the three vertical dots to the right, and select "Redeploy".
-- ![Railway Redeploy option](https://user-images.githubusercontent.com/55474996/226517149-6dc512d5-f862-46f7-a57d-5e55b781ff53.png)
+
+![Railway Redeploy option](https://user-images.githubusercontent.com/55474996/226517149-6dc512d5-f862-46f7-a57d-5e55b781ff53.png)
+
+## Downgrade
+
+To restore a previous version, you have to restore the DB for that particular version. DBs that have been upgraded with a particular version shouldn't be used with older versions. There may be DB changes that a new version brings that are incompatible with previous versions.
+
+**General steps:**
+
+1. Stop listmonk.
+2. Restore your pre-upgrade database.
+3. If you're using docker-compose, edit `docker-compose.yml` and change `listmonk:latest` to `listmonk:v2.4.0` _(for example)_.
+4. Restart.
+
+**Example with docker:**
+
+1. Stop listmonk (app):
+```
+sudo docker container stop listmonk_app
+```
+2. Restore your pre-upgrade db (required) _(be careful, this will wipe your existing DB)_:
+```
+psql -h 127.0.0.1 -p 9432 -U listmonk
+drop schema public cascade;
+create schema public;
+\q
+psql -h 127.0.0.1 -p 9432 -U listmonk -W listmonk < listmonk-preupgrade-db.sql
+```
+3. Edit the `docker-compose.yml`:
+```
+x-app-defaults: &app-defaults
+  restart: unless-stopped
+  image: listmonk/listmonk:v2.4.0
+```
+4. Restart:
+`sudo docker-compose up -d app db nginx certbot`
+