Sony SGPT21 - Manual (Page 65)

Rotating the Screen
The screen rotates when you turn your tablet device sideways shown below.
Note
Depending on the application, the screen may rotate regardless of the direction of your tablet device.
Some apps may not support screen rotation.
Setting the screen to not rotate
1. Tap [ Apps] - [ Settings] - [ Screen] - [Auto-rotate screen] in that order to remove the check mark.
To set the screen to rotate again
Tap [Auto-rotate screen] again to add a check mark.