Will my JellyBean tablet upgrade to KitKat via a download?

If so how to go about it please.

Have you checked for updates? You.should have update software menu.

Tablets and phones and such all need the OEM to provide an update (almost always.) You may be able to install some other OS or version by yourself. It is not recommended for newbies.

Thank you

Further consideration,takes me back 30+ years when early computers ( pre PC)
All had their O/S on ROM's.
To Change O/S out with the soldering iron hours/days of fun with Atari ST,my first.
That's not going to happen with today's devices for me any way , with my arthritic fingers and dimming eyes.

Each product is different. You do have to check online for ideas starting at the company that made the device.

Yes, most of these android images are unique. Some are almost impossible to access to change.

You need to "flash" the ROM. No soldering needed. You back up all data and make a list of all apps downloaded, installed or needed. Sync all possible data (eg: Kindle, evernote). Remember some apps may not permit you to link old data once installed afresh. (Eg:COLORNOTE) In these cases, copy out the data and save it.

Reinstall, reload and go.

OK

Generally vendors lose interest, and just stop doing updates. They are basically interested in getting you to churn to latest-and-greatest hardware, same as phones. That's (more) money for them, whereas free upgrades don't generate anything.

The free community offers updates via custom ROMs (not R/O anymore obviously) for some of the more mainstream/popular tablets. Generic (read Chinese) models tend not to be.
I updated my (old) ASUS TF101 transformer to a custom ROM kitkat - took all weekend, and I wound up with a "bricked" tablet for one of those days. And that was after I'd spent a couple of weeks researching.
Caveat emptor

I guess unlike a regular HDD computer where swapping 0/S and even dual booting is normal(fore some of us) Tablets aew at the stage of early computing where the O/S is on the chip.
The only way to change was change the chip.
Which I did with Atari ST taking out the rooms and replacing with the uupgrade.
Even swapping the Motrola 68000 for a 68300. Lots of soldering 25 years ago

It'll take a brave individual to try that on a Tablet.

No, it's just a barstardised mix of terminology. An Android "ROM" is just an image that has to be loaded onto a partition, similar to loading Linux from an iso. Android is a Linux fork.
The vendors (and to some extent Google) make this as difficult as possible. Generally it requires the open source folks to find an "exploit" (read hack) that enables them to get root privileges so that the update can work. I've hacked my Nexus 5 phone to Android 5 even though an update is not officially available, but in this case it was using Googles factory image. Using "their" hack ...