I am trying to update a label using kubectl v1.18.

I tried kubectl patch deployment my-deployment --patch "$(cat patch1.yaml)"; it returns an error

The Deployment "my-deployment" is invalid: spec.selector: Invalid value: v1.LabelSelector{MatchLabels:mapstringstring{"app":"nginx"}, MatchExpressions:[]v1.LabelSelectorRequirement(nil)}: field is immutable

apiVersion: apps/v1
kind: Deployment
metadata:
  creationTimestamp: null
  labels:
    client: user
  name:  my-deployment
spec:
  replicas: 1
  selector:
    matchLabels:
      app: revproxy
  strategy:
     type: RollingUpdate
     rollingUpdate:
       maxUnavailable: 1
       maxSurge: 2
  template:
    metadata:
      creationTimestamp: null
      labels:
        app: revproxy
    spec:
      containers:
      - image: nginx:1.7.9
        name: nginx
        resources: {}
      dnsPolicy: ClusterFirst
      restartPolicy: Always

The patch yaml is

spec:
  replicas: 3
  selector:
    matchLabels:
      app: nginx
  template:
    metadata:
      labels:
        app: nginx