From the instruction manual:
5. Adjustable output voltage:
The output voltage can be set between 10V and 15V by turning the potmeter (use slotted head screwdriver nr 01 – 3.5mmx0.75mm). The default value is 13,2V. When no load is applied this can be slightly higher. When setting the output voltage always connect a small load. When set with no load the output will be lower when a load is applied. Turn counter clockwise to lower the voltage
The potentiometer is on the same end as the connections, on the right hand side opposite the green remote plug, its in behind the grill and below and to the right of the right hand positive terminal.
I have added a photo to the listing