Заходим на сервер по SSH
Открываем репозитории:
nano /etc/apt/sources.list
Добавляем туда:
deb http://ftp.debian.org/debian bullseye main contrib deb http://ftp.debian.org/debian bullseye-updates main contrib # security updates deb http://security.debian.org/debian-security bullseye-security main contrib deb http://download.proxmox.com/debian/pve bullseye pve-no-subscription
и вводим:
apt update && apt dist-upgrade
Либо:
Datacenter - srv-proxmox - Updates - нажимаем на >_Upgrade