Dnkldorf
Although it isn't impossible that the tapchanger control knob is inside the tank, it's very highly unlikely. It should be somewhere on the outside. It is usually a composite (plastic) knob or lever and should be held in its present position by a set screw or similar. Completely DE-ENERGIZE the transformers before doing anything! ( I know you already know that, but others who happen along to your post may not read everything. I would hate for someone to end up dead, or worse) Previous posts gave the correct instructions. If your present input voltage is 5% lower than the nameplate rated volts, move the tapchanger to the 95% position. Oh, by the way, make really sure you re-tighten the set screw, you would be some surprised what a medium sized curious raccoon can do with that tap handle! Always re-energize from a safe distance whenever possible.
Regards
