Firstly get an overview about all active nodes (nodes to be removed must be running), therefore run the following command on a node that should stay in the cluster:

pvecm nodes

Then shut down the nodes to be removed and delete the node with this command:

pvecm delnode <node-name>

Replace <node-name> with the name of the node that should be removed (take the name from the first command).

When running into issues with the quorum, just set the quorum to 1:

pvecm expected 1

Errors concerned the corosync can be solved by deleting the corosync-conf folder:

rm -rf /etc/pve/priv/lock/file-corosync_conf

Partially sourced from: pve.proxmox.com - PVE admin guide