If you wish to update or upgrade Proxmox, it is probably wise to just look at the Proxmox documentation: https://pve.proxmox.com/wiki/Category:Upgrade. However most of the time, upgrades are fairly simple.

Make sure that the following repository is present:

deb http://download.proxmox.com/debian/pve buster pve-no-subscription

Make sure that you change Buster to the currently running OS, use lsb_release -a

apt update
apt dist-upgrade

Never use apt upgrade , this might break the Proxmox installation and require a complete reinstall!