I want to enable feature gate VolumeSubpathEnvExpansion on a 1.13 kubernetes cluster. I did not found any information about enabling a feature gate for a running cluster.
You can pass --feature-gates= to kubelet. Find systemd's or whatever init system you are using's and edit the kubelet start line to add in VolumeSubpathEnvExpansion and restart kublet service. It is a comma separated list so you might find that your kubelet already has --feature-gates= passed to it with other args.
https://kubernetes.io/docs/reference/command-line-tools-reference/feature-gates/