How do I convert a Lenovo All in One to work as an external monitor?

Most likely no, those are all integrated like a laptop, you can't "just use" the monitor portion on its own. That is the problem with all in one machines, they are basically laptop boards built into a monitor. Not upgradable, not customizable…

Not possible to use that port as it is OUT only, never IN. Only a few HP all in one models have a convertible port.
You can use a computer screen as a monitor by linked software over a local network.
Some choices of those are:
ZoneScreen if your operating systems are old, like WinXP.
MaxiVistA if you can still find it on Internet.
TeamViewer or similar group computing software but this and other like UltraVNC just let you see another computer's desktop screen and use a remote mouse and keyboard. Not for extending the original desktop into wider area.
There are many other ones, but mostly for iOS or Android, that let you use phones and tablets and rarely, computers as extended desktop areas by wireless or (if possible) wired local network. Search Google.
If you are trying to view a laptop because it has no image on the screen (broken?) there's no way to do the linking to another computer without installing software, setting the addresses without viewing on the laptop screen.
Some talented electronics and computer experts have hot-wired old iMacs into DVI type (and HDMI of course) monitor screens. They bypass the motherboard, supply correct power to the image panel and its fluorescent tubes illuminator inverter, install a stub video cable out the case. Quite interesting. Might be possible for an old Windows all-in-one. Mostly it depends upon the pin connections to the panel, power supply, illuminator, and a DVI cable.

You can get converter cables to USB to HDMI adapters

I have a Lenovo All in one, A kludge workaround i use is to send myself an Skype or MS Meeting invite from my laptop to my all-in one gmail and would share my laptop screen onto the Lenovo Screen.