LabQuest 2 has a high-contrast mode for better viewing in sunlight. To activate high-contrast mode tap Home > Preferences > Light & Power, then check the option for “High Contrast”. If necessary you can also increase the screen brightness when on battery power to improve readability of the screen.
For the LabQuest 3 and original LabQuest, generally the screen is bright enough for fieldwork, even at 75% (default) light level. For an even brighter screen, you can choose Power & Light App from Settings (LabQuest 3) or the Control Panel (original LabQuest) and increase the brightness to 100%.