PurpleAir Touch and Zen sensors can be double-tapped to cycle through different brightness options.
The taps have to be spaced out correctly for the mechanism to trigger. If the brightness does not change, try tapping faster or slower.
It is possible that placing something down or bumping the counter a sensor is on can activate the double-tap mechanism. If the light on your device is dimmed or off, try double-tapping it to change the current setting.