In order to run a manual command against etcd running inside the K8S cluster you have to pass a few flags
Be very careful with this as you can really mess things up
Get the members of the etcd cluster

kubectl exec -it -n kube-system etcd-k8sn1 -- etcdctl --cacert=/etc/kubernetes/pki/etcd/ca.crt --cert=/etc/kubernetes/pki/etcd/peer.crt --key=/etc/kubernetes/pki/etcd/peer.key --endpoints=https://127.0.0.1:2379 member list

Should get you output like:

3c06d56f4e8193e0: name=k8sn3 peerURLs=https://192.168.1.54:2380 clientURLs=https://192.168.1.54:2379 isLeader=false
3cc1193e53313d05: name=k8sn2 peerURLs=https://192.168.1.55:2380 clientURLs=https://192.168.1.55:2379 isLeader=false
a7447908c6035378: name=k8sn1 peerURLs=https://192.168.1.52:2380 clientURLs=https://192.168.1.52:2379 isLeader=false
efd47b5d63640666: name=k8sn4 peerURLs=https://192.168.1.53:2380 clientURLs=https://192.168.1.53:2379 isLeader=true

You can then remove a specific node

kubectl exec -it -n kube-system etcd-k8sn1 -- etcdctl --cacert=/etc/kubernetes/pki/etcd/ca.crt --cert=/etc/kubernetes/pki/etcd/peer.crt --key=/etc/kubernetes/pki/etcd/peer.key --endpoints=https://127.0.0.1:2379 member remove a7447908c6035378