Taps are on the primary side. If your voltage is low you need to select a lower voltage tap (take some of the primary winding out of the circuit). If your voltage is high you need to select a higher voltage tap (add some primary winding to the circuit) It is just the opposite of what you think you would do . It's all about turns raito. If you have a chance to look at a high voltage transformer like 13200/480V it should have a nameplate with a diagram it will be easy to see what is happining with the primary taps.
