First, go to my.meural.com, log in, and select your Canvas on the top left. Under "Settings", scroll down to "Auto-adjust brightness" and make sure it's set to "On."
If it is set to "On", go to the Canvas, wave down, and select "Canvas info." The "Light Sensor" shows the light detected by the Canvas. If the room isn't dark, and the value is less than 15, change the "Light Sensitivity" setting on my.meural.com to "High." If the value is more than 200, change the "Light Sensitivity" to "Low."
You can also adjust this setting on the Meural App, On the Meural App, go to the Canvas Settings, scroll down to "Auto-adjust brightness".