"大灯变光开关" can be translated as "headlamp beam selector switch" or more simply as "headlight dimmer switch".
"Headlamp beam selector switch" emphasizes the function of selecting between different beams (such as high beam and low beam) of the headlamps.
"Headlight dimmer switch" is a more colloquial expression, where "dimmer" refers to the device that adjusts the brightness of the headlights, though in this context, it's more about switching between beams rather than continuously adjusting brightness.
Both terms are commonly used in automotive contexts to refer to the switch that controls the change between high beam and low beam headlights.