Use hardware interrupt.
syntax: attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);
ISR: write the function name (in where it goes whenever you press pushbutton, where your increment/decrement happens)
mode: defines when the interrupt should be triggered. Four constants are predefined as valid values:
RISING : to trigger when the pin goes from low to high,
FALLING : for when the pin goes from high to low.
check for your arduino (UNO/Mega/...) interrupt pins.
Hope it helps. ping back, if not.