Based on your post, I assume that you want to move the servo a small amount on each press of a button which would not require you to interact with the changeRate which controls how quickly the servo moves to a new position.
The basic approach I've seen to this is to set the servo target to the current value plus an incremental step.
Please note that I have not tested the code above.
Also note that care is taken to avoid moving the servo beyond the intended range. Another thing you will have to contend with is holding the button down may shoot through the loop and end up instantly setting you to your min or max targets. You may have to either use a variable to control moving in and out of the if statements or move the code into another task with a delay.