How to upgrade Kubectl version on MacOs High Sierra (version 10.13.6)
Before the upgrade :
$ kubectl version –short
Client Version: v1.11.3
Server Version: v1.10.5
Downloaded Latest Kubectl
$ curl -LO https://storage.googleapis.com/kubernetes-release/release/$(curl -s https://storage.googleapis.com/kubernetes-release/release/stable.txt)/bin/darwin/amd64/kubectl
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 46.3M 100 46.3M 0 0 5913k 0 0:00:08 0:00:08 –:–:– 6743k
Modify File Permission
$ chmod +x ./kubectl
Move Kubectl to /use/local/bin
$ sudo mv ./kubectl /usr/local/bin/kubectl
After the Upgrade
$ kubectl version –short
Client Version: v1.15.0
Server Version: v1.10.5

Refer : https://kubernetes.io/docs/tasks/tools/install-kubectl/#install-kubectl-on-macos