How to change the prometheus scraping interval?

By default the scraping interval is set to 60s:
kc -n cattle-prometheus exec -it prometheus-cluster-monitoring-0 ‘sh’
Defaulting container name to prometheus.
Use ‘kubectl describe pod/prometheus-cluster-monitoring-0 -n cattle-prometheus’ to see all of the containers in this pod.
/prometheus $ head /etc/prometheus/config_out/prometheus.env.yaml
evaluation_interval: 60s
scrape_interval: 60s
prometheus: cattle-prometheus/cluster-monitoring
prometheus_from: cluster-dev0
prometheus_replica: prometheus-cluster-monitoring-0

How do I change it?

This does the trick for cluster level monitoring:
In the Tools->Monitoring page of your cluster, click on ‘show advanced options’ and add the following answer:
prometheus.scrapeInterval = <whatever_value_you_want>