First, note the node IDs
On manager node:
docker node update --availability drain $WORKER_ID
This is optional
docker swarm join-token manager
This command will give you the join command to run on each node after it's removed. I'll refer to it as $JOIN_COMMAND below. We will demote the worker once the manager re-joins.
docker swarm leave
This node is now re-joined as a manager, but I'll continue calling it the 'worker' to avoid confusion.
docker node rm $WORKER_ID
docker node update --availability drain $MANAGER_ID
docker swarm leave -f
docker node rm $MANAGER_ID
docker node ls
Find the worker's new id (pay attention to the hostname, not the role) -> $NEW_WORKER_ID
docker node demote $NEW_WORKER_ID
Your swarm should be refreshed - if there were more nodes, the services running on each would have migrated across the swarm when you drained each node.