add information about upgrading RDS postgres engine

docs/concepts/managed-storage/managed-postgres.md

@@ -68,3 +68,21 @@ You can connect to the managed Postgres instance using the name of your service
       // highlight-end
 
 ```
+
+### Major Version Updating of Engine
+
+To update the database engine you can simply update the image to a later version in your compose file and apply it via ```defang compose up --provider=aws```. In the example below, we change from Postgres 15 to 16.
+
+Please note the upgrading will occur immediately and may result in the database being unavailable for some time.
+
+```
+database:
+  image: postgres:15
+```
+
+to
+
+```
+database:
+  image: postgres:16
+```