The problem:

apt-get dist-upgrade 

E: Failed to fetch https://enterprise.proxmox.com/debian/pve/dists/buster/InRelease 401 Unauthorized [IP: 66.70.154.81 443]
E: The repository ‘https://enterprise.proxmox.com/debian/pve buster InRelease’ is not signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.

Quick fix:

cd /etc/apt/sources.list.d/ 
cp pve-enterprise.list pve-no-subscription.list 
nano pve-enterprise.list

Comment out the first line and save:
#deb https://enterprise.proxmox.com/debian/pve buster pve-enterprise

nano pve-no-subscription.list

change deb https://enterprise.proxmox.com/debian/pve buster pve-enterprise
to deb http://download.proxmox.com/debian/pve buster pve-no-subscription

Upgrade should work now.

Reference: https://www.caretech.io/2018/06/08/how-to-update-proxmox-without-buying-a-subscription/

EDIT: Found a nicer way to do it: https://gist.github.com/whiskerz007/53c6aa5d624154bacbbc54880e1e3b2a
This script will not remove nagging popup with the newer versions of Proxmox, works up to 5.3.x version.

For a newer versions use this command:

 sed -i.bak "s/data.status !== 'Active'/false/g" /usr/share/javascript/proxmox-widget-toolkit/proxmoxlib.js && systemctl restart pveproxy.service

Make sure to clear your cookies and cache for this method to work.