mkdir /mnt/iso
mount -t iso9660 -o loop /var/lib/vz/template/iso/proxmox-ve_7.3-1.iso /mnt/iso
nano /etc/apt/sources.list.d/
nano /etc/apt/sources.list.d/pve-enterprise.list 
 
deb [trusted=yes] file:///mnt/iso/ bullseye pve
deb [trusted=yes] file:///mnt/iso/proxmox/packages/ /
 
apt update & apt dist-upgrade