If i understand correctly, exposure is the measure of lightness in a picture. In my phone, there are no settings by which I can set it manually (it does it automatically), I use to put my hand on camera and then remove it suddenly and take a shot to get higher exposure when needed. For low exposure I use to face camera towards a lights source then suddenly back to the point and take a shot.
This is an ugly way to do it, is there any software or technique which can do that??