Big, red-colored "Demo" text on the center of the tablet screen The word "Demo" is visible in the center of the screen in a big red font at all times. It is there even when you play games or go to Play Store. The tablet is not a floor model nor in demo mode.

   
Screenshots (click images for larger variants)

How do I remove the "Demo" text from the device screen?
Precaution: Trojan reported in this file by MalwareBytes Mobile (from comment). Use at your own risk.

From YouTube video posted on Jose's comment,


Turn on the device.
Connect to your PC by the USB.
Download SystemPack.apk (original: MEGA, mirror: MediaFire).
Copy the .apk file to a directory on your device's memory.
On your device, look for the .apk file using the files manager and install it by touching on it and following the given instructions.
Shutdown your device, wait for 30 seconds, and the &quot;DEMO&quot; text will be gone.

Notice that the .apk file is like a &quot;patch&quot;, so if you reset your phone into factory settings, the &quot;DEMO&quot; text will appear again.

Disclosure: The MediaFire mirror was uploaded by original poster of the answer. The file was uploaded as-is, without any compromise.