A typical way to add buttons is to connect one side to ground and the other to one of the arduino pins and then define that pin as INPUT_PULLUP to enable the internal pullup resistor. You have to read the state of the arduino pin and if it is HIGH, it is not being pushed, if it is LOW, it is being pushed. Also do some research on "debounce" for the button pushes (or use the Button library)
Based on which button is being pushed, you add or subtract 1 from your setpoint, making sure you stay within your range.
Give it a shot and post your code if you have problems.