Android - 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!