Using kubectl requires the installation of kubectl on your system as well as downloading of kubeconfig on the cluster UI page. See the Official kubectl Install Instructions for a tutorial on how to install kubectl on your system. Once you have installed it, download the kubeconfig. The below steps will guide you on how to download a kubeconfig.
To download the kubeconfig, navigate to Clusters and select the correct cluster. On the top right you can find a download button circled with red box:
You can revoke access for already downloaded kubeconfigs by revoking the token on the cluster detail page. To do so, click on the three-dot settings icon on the right to see the option Revoke Token:
Users in the groups Owner and Editor have an admin token in their kubeconfig. Users in the group Viewer have a viewer token. Revoking the token for a user group means the kubeconfig becomes unusable for users in this group and they need to download it again. Using kubectl with the invalid kubeconfig will result in an error message. You can see which group every project member belongs to on the Members page.
Once you have installed the kubectl and downloaded the kubeconfig, change into the download directory and export it to your environment:
$ export KUBECONFIG=$PWD/kubeconfig-admin-czmg7r2sxm
$ kubectl version
Client Version: version.Info{Major:"1", Minor:"13", GitVersion:"v1.13.0", GitCommit:"...", GitTreeState:"clean", BuildDate:"...", GoVersion:"go1.11.2", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"14", GitVersion:"v1.14.8", GitCommit:"...", GitTreeState:"clean", BuildDate:"...", GoVersion:"go1.12.10", Compiler:"gc", Platform:"linux/amd64}