You can overwrite the default Alertmanager configuration by editing the alertmanager-main secret inside the openshift-monitoring namespace.

  1. Print the currently active Alertmanager configuration into file alertmanager.yaml: 
  oc -n openshift-monitoring get secret alertmanager-main --template='{{ index .data "alertmanager.yaml" }}' |base64 -d > alertmanager.yaml

  1. Change the configuration in file alertmanager.yaml to your new configuration

  2. Apply the new configuration in the file:

  oc -n openshift-monitoring create secret generic alertmanager-main --from-file=alertmanager.yaml --dry-run -o=yaml | oc -n openshift-monitoring replace secret --filename=-