Upgrade from 2.1 to 2.3

Hello,

what must I do that I can update Proxmox from Version 2.1 to Version 2.3 ?
Actually I have a cluster with 3 Servers (Version 2.1) and a new which have Version 2.3

Now I want to update the 3 Servers to 2.3 that I can put the new one in the same cluster

Regards