The easiest way to do a clean re-install of the mgmtcompanion is to first delete it's namespace:

sudo $(which kubectl) --kubeconfig /etc/rancher/k3s/k3s.yaml delete namespace mgmtcompanion

And then use the webinterface to create a new instance, and execute the generated command. Note: It will ask you if you want to overwrite your UMH Helm Chart, which you should decline (this is the default option), otherwise it will remove your data !