For the feedback path to the mains switching side, there is usually a voltage reference on the secondary side that is compared to a divided voltage of the output.
To adjust the output voltage, either change the reference voltage (if it's adjustable) or the divider ratio.
As tunk mentioned, TL431s are common voltage reference ICs.
To get a start, you could scope the secondary side of the opto-isolators to see which one is responsible for transmitting this "too high, too low" signal.