You can use kubectl patch command to update K8s resources.

kubectl patch configmap/test \
  --type=json \
  '-p=[{"op": "replace","path": "/data/key1", "value": "test1"}]'

An important point to note as mentioned by Henry is that the application also has to re-read the properties after they have been changed.

You can use Bash script to dynamically replace some keys and values in your ConfigMaps.

I've created simple bash script to illustrate how it may works on my kubeadm cluster v1.20:

#!/bin/bash

keyName="key1"
value="value100"

read -p 'Enter ConfigMap name: ' configmapName

if kubectl get cm ${configmapName} 1> /dev/null 2>&1; then
        echo "ConfigMap name to modify: ${configmapName}"
else
        echo "ERROR: bad ConfigMap name"
        exit 1
fi

kubectl patch cm ${configmapName} -p "{\"data\":{\"${keyName}\":\"${value}\"}}"

In the above example you need to pass ConfigMap name and set what you want to modify.<br> In addition you may want to pass keyName and value values as command line arguments in similar way to configmapName value.

You can see an example of how the above script works:<br>

root@kmaster:~# ./replaceValue.sh 
Enter ConfigMap name: test
ConfigMap name to modify: test
configmap/test patched
root@kmaster:~# kubectl describe cm test
Name:         test
Namespace:    default
Labels:       <none>
Annotations:  <none>

Data
====
application.properties:
----

key1:
----
value100
key2:
----
value2
keyN:
----
valueN
Events:  <none>
root@kmaster:~# 

Note: If you want to use kubectl replace instead of kubectl patch, you can use below command (e.g. sourceValue="key1: value1" and destinationValue="key1: value100")<br>

kubectl get cm ${configmapName} -o yaml | sed "s/${sourceValue}/${destinationValue}/" | kubectl replace -f -