A new lab has been added to Proxmox VE 8: Advanced Administration and Optimization that teaches the upgrade to Proxmox VE 9 step by step: Lab 15, Upgrading a Proxmox Cluster to PVE 9, takes a VE 8 cluster to VE 9.
A single-server companion lab, Upgrading a Proxmox Node to PVE 9, is in development for Proxmox VE 8: Setup and Management.
Rehearsing the upgrade in a lab surfaces the version-specific steps (repository changes, package transitions, node sequencing in a cluster) before you run them against systems your classes depend on.
On NDG Online
The lab is live now in Proxmox VE 8: Advanced Administration and Optimization, with nothing to set up.
On NETLAB+
The same lab ships in the VE 8 lab library: Advanced Administration and Optimization. Update the course in the Course Manager to Release 3 to receive Lab 15; details are in the Proxmox content release notes.
Upgrading your own system
The lab teaches Proxmox upgrade skills; NETLAB+ administrators upgrading their system's Proxmox backend should follow the NETLAB+ guide to upgrading Proxmox VE 8 to 9, which covers preparation, the upgrade itself, and post-upgrade checks one node at a time. The upgrade path to Proxmox VE 9 also links the Proxmox VE 9: Setup and Management course for teaching the new version.