You de-energize it first. What ever you do, don't change taps under load!
Once it's de-energized, just move the dial to correspond to the percentage tap you want. You'll notice they are in 2.5% increments. If your output is 5% low on the 100% tap, go to the 95% tap. Essentially, you will be applying whatever the original incoming voltage was across 95% of the windings. The volts per winding will be higher on the primary, so the volts per winding on the secondary will be too. Do all three pots the same and double check your output before applying load.
By the way, I do business with Solomon quite often and find them to be very helpful and professional. I'm sure they would be willing to answer any questions you had about their equipment.
[This message has been edited by WFO (edited 04-03-2006).]
[This message has been edited by WFO (edited 04-03-2006).]