In the first place, wake up the screen on your XIAOMI Mi Watch Lite by pressing the Side button.
Then open the Quick Panel by swiping upwards from the bottom of the screen.
Now tap on the Settings icon.
Afterward, tap on the Brightness option.
Now if the Auto Adjust option is enabled, disable it by tapping on the switch next to it.
After that select Brightness.
Here find the level of Brightness you like and tap on it to apply it.
Source : hardreset.info