Kubernetes integration: trouble understanding where is documentation to configure the new runner

On private Gitlab 12.9.2, I add my own kubernetes cluster to my Gitlab project.
I install Helm and Gitlab runner on the cluster, my goal is to run CI/CD pipelines.
Now, I can see a new kubernetes runner is installed in the CI/CD/runners tab of the project.

But I don’t understand how to configure the runner. I want to change its executor to docker, but how to modify its config, as it is running in kube?

It may be obvious but there are so many ways to install runners in the documentation, I don’t know how my installation is referred to. Am I in the usecase of Installing GitLab Runner using the Helm Chart ? But I did not use the Helm CLI, I used the Gitlab webapp GUI directly.