How to change the demo OS in Sony xperia tablet z
Since you don't have a dialer app in the tablet, Ali Kabiri's answer can't be directly used. But there's a way to get into a mode where you can enter these "service codes". See this blog post for images.


Open the Contacts app
Exit the initial setup if it pops up
Go to Menu -> Settings
Long-press the blank space in the bottom of the screen, in the middle (see images in link)
A button called "Enter service code" should appear
Tap the button to get a "dialer"
Enter the code *#*#73556673#*#*
Demo mode should now be deactivated!