TL;DR
$ helm repo add prometheus-community https://prometheus-community.github.io/helm-charts
$ helm repo update
$ helm show values prometheus-community/kube-prometheus-stack
$ helm show values prometheus-community/kube-prometheus-stack > values.yaml
# Edit values.yaml
$ helm install prometheus-community prometheus-community/kube-prometheus-stack --namespace monitoring -f values.yaml --create-namespace
# update values.yaml
$ helm upgrade --install prometheus-community prometheus-community/kube-prometheus-stack --namespace monitoring -f values.yaml