#Step 1. stop the cluster file system in /etc/pve/
# service pve-cluster stop

#or if you use PVE 4.0 and newer
 systemctl stop pve-cluster

#Step 2. start it again but forcing local mode
 pmxcfs -l

#Step 3. remove the cluster config
 rm -f /etc/pve/cluster.conf /etc/pve/corosync.conf 
 rm -f /etc/cluster/cluster.conf /etc/corosync/corosync.conf 
 rm /var/lib/pve-cluster/corosync.authkey

#Step 4. stop the cluster file system again

# - on PVE 3.4 and earlier
# service pve-cluster stop

# -  on PVE 4.0 and newer
 systemctl stop pve-cluster

#Step 5. (optional) you may have to delete the lockfile of the cluster filesystem:
 rm /var/lib/pve-cluster/.pmxcfs.lockfile

#Step 6. restart pve services (or reboot)

# - on PVE 3.4 and older:
# service pve-cluster start
# service pvedaemon restart
# service pveproxy restart
# service pvestatd restart

#- on pve 4.0 and newer
 systemctl start pve-cluster
 systemctl restart pvedaemon
 systemctl restart pveproxy
 systemctl restart pvestatd