add cluster upgrade to docs

docs/docs/workflows/terraform-module.md

@@ -3,7 +3,7 @@ It's possible to manage Constellation, including upgrades, through Terraform.
 The necessary module package is available as part of the release and consists of a convenience module for each cloud service provider (`{csp}-constellation`) that combines the IAM (`infrastructure/{csp}/iam`), infrastructure (`infrastructure/{csp}`) and constellation (`constellation-cluster`) modules.
 
 ## Quick setup
-The convenience module allows to set up a Constellation cluster with a single module including all configurability that is otherwise possible through the CLI. It's easiest to consume the module through a remote source, as shown below.
+The convenience module allows to set up a Constellation cluster with a single module including all configurability that's otherwise possible through the CLI. It's easiest to consume the module through a remote source, as shown below.
 This allows to upgrade the cluster to a newer Constellation version by simply updating the module source.
 
 In all examples remember to replace `$VERSION`.
@@ -75,3 +75,10 @@ The submodules are:
 - `fetch-image`: used to translate the Constellation image version to the image ID of the cloud service provider
 - `infrastructure/{csp}`: contains the cluster infrastructure resources
 - `infrastructure/iam/{csp}`: contains the resources to set up IAM
+
+## Cluster upgrades
+using the remote source:
+1. update the module source
+2. update the image / Kubernetes version (if explicitly set)
+3. `terraform init --upgrade`
+4. `terraform apply`