There are three methods.

[Change the setting on V9 series]
Switch to Local mode, select [System setting] > [Backlight setting] to make changes and press [Apply] button.

To restore the screen data setting, press [Restore Screen Data Setting] > [Apply].

[Change the setting in the screen program]
Setting location: [System Setting] > [Unit Setting] > [Backlight]

[Change the setting on V9 while it is in RUN mode]
It can be changed by using macro command "SYS(SET_BKLT)".
For more information, click here.

For detailed setting, refer to "How to make the screen dark when it is left untouched for a preset time".