Hypernym for "increase" and "decrease"

Is there a word in English which would encompass both the action of increasing or decreasing [the value of] something?

I am looking at a word similar to modify, change or alter. But the first two do not denote the fact that the current value would be slightly changed and I feel the the last one is a bit general.

One such word (and one commonly used by scientists) is vary.

Like increase and decrease, it is not explicit whether vary is an action performed on the variable, or something that the variable "just does".

You should vary the input voltage and observe the result.

variation

Typically in technical usage, variation represents change: variations in voltage, current, so on. Moreover, while variation can mean drastic changes, it is typically associated with small differences in state, discrete or continuous.

If you specifically want to imply that the changes are small, vary is not entirely apt. I'd go with tweak "a fine adjustment to a mechanism or system" instead. Not to be confused with twerk, of course.