My 7" Android 4.4 kitkat tablet just recently started to display the word "DEMO" in big red letters on every screen. How do I get rid of it?

Factory reset it,

Power off completely
2. then hold down the volume down button
3. holding the button press the power on button.
4. if done properly you should have several android boot options
5. select "Factory data reset"
6. Done

Note: if the volume down did not work, try the Volume Up button

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Return it to where you bought it, someone has customised the start up screen.

Turn it off demo mode in the settings. Your user manual will have details on how to do this

Try the procedure at the bottom of this web page SOLVED How to Remove Demo from the Zeepad Screen Android Tablet Forum

below is a link if you read #14 this will give you instructions on how to install the software that has removed it i have not registered so i can not confirm the software works.you will need to down load a file called SystemPack.APK and run it from your tablet as ypu would a app

If there is an options menu go and find the demo there should be one. I think that would be the place. Hope this helped you.

Please try this:
Tap on Apps in home screen - Settings - Storage - Internal Storage - Apps - Scroll down and Identify "Demo Video Application" under the apps list .

Tap on Clear Cache and tab on "Disable", Ignore the warning message.
Note: Cust may see the "Demo mode" icon in the homescreen, however it wont bringup Demo Video

You might find the answer here:
ARCHOS 101 User Guide

Where did you get the tablet? I believe you purchased a store demo tablet. If you you browse zeepad's website you can probably restore the firmware and get rid of store demo mode.