So instead, we'll use the "mintupdate" GUI on the Remote (headless) Desktop, via ssh, which worked for me, over my internal LAN. (see Note 3: at bottom).
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Requirements:
-The Remote Mint Desktop must have a Mint 21.2 Desktop environment with the "openssh-server" pkg installed and SSHD enabled and running.
-It can be Headless,-(no Monitor, no Keyboard, no Mouse), but with the standard Linux Mint Xorg libs, ... installed).
RMD = IPADDRESS of Remote Mint 21.2 Desktop
LMD = IPADDRESS of Local Mint 21.* Desktop
1./
From LMD ssh into RMD:
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sudo apt update
sudo apt upgrade
sudo reboot
From LMD ssh back into RMD:
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sudo cp /etc/ssh/sshd_config /etc/ssh/sshd_config.orig
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sudo vi /etc/ssh/sshd_config
(and make sure the lines, PermitRootLogin, and X11Forwarding are set to yes)
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PermitRootLogin yes
X11Forwarding yes
3./
From LMD ssh again into RMD:
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ssh -X -v $USER@RMD
export XAUTHORITY=$HOME/.Xauthority
sudo mintupdate
Update Manager -> Edit -> Software Sources.
Sofware Sources -> Official Repositories -> select "Restore the default settings", then click OK to update APT cache.
Sofware Sources -> PPAs -> disable any 3rd-party PPAs, ..., for this Upgrade, then close the Software Sources window.
-(you can re-Add/Enable those PPAs after the Upgrade).
Update Manager -> Refresh.
Update Manager -> Edit -> Upgrade to "Linux Mint 21.3 Virginia".
...complete the Upgrade process...and Reboot.
see Note 2: below.
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Note 2: See post-Upgrade Reference information here: https://blog.linuxmint.com/?p=4629
Note 3: If the Remote Mint 21.2 Desktop, that you want to upgrade, is external from your LAN/aka across the Internet, then obviously, a successful sshd connection, and X display back to local, will depend on the external Router/Firewall and the open sshd-enabled port(s)/virtual-server/trigger-over-NAT configurations, ...?, agreed upon, which is beyond this little tutorial, but yes, it can easily be accomplished.
Or, just use a (remotely and locally configured) Teamviewer app, or similar, but in either case you'll still need access to the Remote Mint Desktop.
In the long run, I feel SSH is still a much more efficient, extremely light weight, secure, free\opensource built-in solution, for ALL Unix-based networked OS's.