Anhand dieser Doku sollte ein Update von PVE Version 6.x zu 7.x durchführbar sein.
Doku von Proxmox selbst: https://pve.proxmox.com/wiki/Upgrade_from_6.x_to_7.0

mkdir -p /root/backup/`date '+%Y%m%d'`
tar -czf /root/backup/`date '+%Y%m%d'`/pve-cluster-backup.tar.gz /var/lib/pve-cluster
tar -czf /root/backup/`date '+%Y%m%d'`/ssh-backup.tar.gz /root/.ssh
tar -czf /root/backup/`date '+%Y%m%d'`/corosync-backup.tar.gz /etc/corosync
cp /etc/hosts /root/backup/`date '+%Y%m%d'`/
cp /etc/network/interfaces /root/backup/`date '+%Y%m%d'`/

Auf diesen gehe ich nicht mehr ein da man diesen sowieso nicht mehr verwenden sollte.

apt-get update
apt-get dist-upgrade
reboot

PVE bringt wieder Upgrade Check mit. Mit diesem kann man checken lassen ob ein Upgrade möglich wäre.

Beispiel:
root@pve03:~# pve6to7
= CHECKING VERSION INFORMATION FOR PVE PACKAGES =

Checking for package updates..
PASS: all packages uptodate

Checking proxmox-ve package version..
PASS: proxmox-ve package has version >= 6.4-1

Checking running kernel version..
PASS: expected running kernel '5.4.124-1-pve'.

= CHECKING CLUSTER HEALTH/SETTINGS =

SKIP: standalone node.

= CHECKING HYPER-CONVERGED CEPH STATUS =

SKIP: no hyper-converged ceph setup detected!

= CHECKING CONFIGURED STORAGES =

PASS: storage 'Storage-SSD' enabled and active.
PASS: storage 'local' enabled and active.
PASS: storage 'local-lvm' enabled and active.

= MISCELLANEOUS CHECKS =

INFO: Checking common daemon services..
PASS: systemd unit 'pveproxy.service' is in state 'active'
PASS: systemd unit 'pvedaemon.service' is in state 'active'
PASS: systemd unit 'pvestatd.service' is in state 'active'
INFO: Checking for running guests..
PASS: no running guest detected.
INFO: Checking if the local node's hostname 'bowser' is resolvable..
INFO: Checking if resolved IP is configured on local node..
PASS: Resolved node IP '86.59.94.247' configured and active on single interface.
INFO: Checking backup retention settings..
INFO: storage 'local' - no backup retention settings defined - by default, PVE 7.x will no longer keep only the last backup, but all backups
PASS: no problems found.
INFO: checking CIFS credential location..
PASS: no CIFS credentials at outdated location found.
INFO: Checking custom roles for pool permissions..
INFO: Checking node and guest description/note legnth..
PASS: All node config descriptions fit in the new limit of 64 KiB
PASS: All guest config descriptions fit in the new limit of 8 KiB
INFO: Checking storage content type configuration..
PASS: no problems found
SKIP: NOTE: Expensive checks, like CT cgroupv2 compat, not performed without '--full' parameter

= SUMMARY =

TOTAL:    19
PASSED:   16
SKIPPED:  3
WARNINGS: 0
FAILURES: 0