$ cat secret.yaml
apiVersion: v1
kind: Secret
data:
  foo: bar
  key: value
$ k8s-secrets-decode -encode -file secret.yaml
apiVersion: v1
data:
  foo: YmFy
  key: dmFsdWU=
kind: Secret  

kubectl get secrets secret123 -o yaml | k8s-secrets-decode

$ k8s-secrets-decode -help
Usage of ./k8s-secrets-decode:
  -encode
    	Encode instead of decode
  -file string
    	Read from file instead of stdin
  -output-file string
    	Write output to file instead of stdout