How do you override default values in Helm charts?

You pass your own values with --set flags or values files at install or upgrade time. Helm merges them over the chart's defaults in a fixed order of precedence, highest first:

1. --set on the command line wins over everything.
2. A custom values file passed with --values, or its short form -f.
3. When you pass several -f files, a later file overrides an earlier one.
4. The chart's own values.yaml supplies the defaults and has the lowest precedence.

helm install myapp ./mychart --set replicaCount=5 --set image.tag=2.0

helm install myapp ./mychart --values custom-values.yaml

helm install myapp ./mychart -f values.yaml -f production-values.yaml

A custom values file only needs the keys you want to change:

replicaCount: 3
image:
  repository: myapp
  tag: "v2.1.0"
service:
  type: LoadBalancer
resources:
  limits:
    memory: "512Mi"
    cpu: "500m"

Helm merges maps deeply, so setting image.tag keeps the default image.repository. Arrays are replaced whole, so to change one item you pass the full list.