Braitenberg vehicle, change modes

Hi guys.. I have to do Braitenberg vehicle with changing modes.
I want to code "change" from this

leftServo.write(90 + rightValue);
rightServo.write(90 - leftValue);

to this

leftServo.write(90 - rightValue);
rightServo.write(90 + leftValue);

with button.
Is there a any willing person to help me how to get the buttons to arduino and how code will looks like or send me some links?
(I already have working engagement, with no buttons)

Open up your Examples folder. Find the "state change" example. Get that working with a button and LED first.