Login in your Proxmox via SSH or use the GUI Shell.

nano /etc/apt/sources.list

Add at the bottom of that file

# PVE pve-no-subscription repository provided by proxmox.com,
# NOT recommended for production use
deb http://download.proxmox.com/debian/pve bullseye pve-no-subscription

We’ll also disable the enterprise updates as we can’t access them anyway

nano /etc/apt/sources.list.d/pve-enterprise.list

Then comment out the line by placing a # at the beginning

Now run the following command to update the server with the latest package details

apt update
apt full-upgrade