In the Options > Accessibility menu, you can adjust the size of the on-screen controls, and activate or deactivate the enhanced brightness. 

You can also choose between a fixed or floating joystick (visible or invisible), as well as activating automatic movement and hiding the up/down buttons for swimming.

Finally, you can choose a button style and adjust the camera speed.

Please note that you can’t change the location of the buttons on the screen, apart from the joystick.