I'm guessing you mean the modifier's effects (like +1 armed forces approval, or -10% infrastructure, etc.). The effect to add a modifier can specify a multiplier that multiplies all effects of the given modifier while it's active (even with negatives to invert the effects), but otherwise the individual effects of a given modifier can't be changed.
Similarly, you can't add or remove effects to a modifier directly, but instead need to remove the current modifier and add another with the new effects.