It’s probably not a good idea for actual use, but if you’d like to experiment: looks like the 3-pin devices U4 and U5 on the upper right provide feedback through the optocouplers next to the class Y cap north of the transformer. I bet those are LM431 voltage references (or similar). The passives around them provide filtering, but two of the resistors should form a resistor divider for the Ref pin (lower right pin if the single pin is on top). That divider sets the voltage.