How can I edit a Deployment without modify the file manually?

I have defined a Deployment for my app:

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: myapp-deployment
spec:
  replicas: 2
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
      - name: myapp
        image: 172.20.34.206:5000/myapp_img:2.0
        ports:
        - containerPort: 8080

Now, if I want update my app's image 2.0 to 3.0, I do this:

1. $ kubectl edit deployment/myapp-deployment
2. vim is open. I change the image version from 2.0 to 3.0 and save.

How can it be automated? Is there a way to do it just running a command? Something like:

$ kubectl edit deployment/myapp-deployment --image=172.20.34.206:5000/myapp:img:3.0

I thought using Kubernetes API REST but I don't understand the documentation.

You could do it via the REST API using the PATCH verb. However, an easier way is to use kubectl patch. The following command updates your app's tag:

kubectl patch deployment myapp-deployment -p \
  '{"spec":{"template":{"spec":{"containers":[{"name":"myapp","image":"172.20.34.206:5000/myapp:img:3.0"}]}}}}'

According to the documentation, YAML format should be accepted as well. See Kubernetes issue #458 though (and in particular this comment) which may hint at a problem.

There is a set image command which may be useful in simple cases

Update existing container image(s) of resources. Possible resources include (case insensitive): pod (po), replicationcontroller (rc), deployment (deploy), daemonset (ds), job, replicaset (rs)

kubectl set image (-f FILENAME | TYPE NAME) CONTAINER_NAME_1=CONTAINER_IMAGE_1 ... CONTAINER_NAME_N=CONTAINER_IMAGE_N

http://kubernetes.io/docs/user-guide/kubectl/kubectl_set_image/

$ kubectl set image deployment/nginx-deployment nginx=nginx:1.9.1
deployment "nginx-deployment" image updated

http://kubernetes.io/docs/user-guide/deployments/

(I would have posted this as a comment if I had enough reputation)

Yes, as per http://kubernetes.io/docs/user-guide/kubectl/kubectl_patch/ both JSON and YAML formats are accepted.

But I see that all the examples there are using JSON format. Filed https://github.com/kubernetes/kubernetes.github.io/issues/458 to add a YAML format example.