Prerequisite:
You need to know how to SSH.
and/or
Need to know how to work in the Linux terminal.
Basic Linux knowledge.

Since I didn’t pay for the subscription I can’t get the updates from it.
But what I can do, is adding repositories instead.

I will do it from the terminal, but it can be done from the WebUI as well.

There’s two ways of doing it.
Through the nano text editor, or just echo the lines into the file.

The nano way.

1. Edit the list of sources:
   nano /etc/apt/sources.list
2. Add the following lines at the bottom of the list:
   # 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

3. Disable the enterprise by adding # at the beginning.
   nano /etc/apt/sources.list.d/pve-enterprise.list

The echo way.

1. Echo the lines:
   sh -c 'echo "# VE pve-no-subscription repository provided by proxmox.com," >> /etc/apt/sources.list'
sh -c 'echo "# NOT recommended for production use" >> /etc/apt/sources.list'
sh -c 'echo "deb http://download.proxmox.com/debian/pve bullseye pve-no-subscription" >> /etc/apt/sources.list'
2. Adding the # to the subscription line:
   sh -c 'echo "#" > /etc/apt/sources.list.d/pve-enterprise.list'

To update your system use:
apt upgrade && apt full-upgrade -y

You can reboot the system if you want. But it should tell you if it needs to.