Push button (about 1 cm in diameter or more)
Bumper sensors basically are just a button with a larger piece (the bumper) attached to it. Most buttons are not glued together but are snapped in place. The snap might either be part of the casing or the button itself. Use a screwdriver or whatnot to open the button. Do so over a very clean floor or desk as they can have very small parts inside waiting to fly out.
There are three basic types of buttons: one spring, two spring and no spring buttons. The no spring types are what you would see on a calculator or keypad and use a clear rubber for springiness. They already require such little force that if you were using them, you would probably not need this tutorial. Two spring buttons most likely hold a contact plate between the springs. You want to modify the bottom spring and not the top. For one spring buttons, you only have one spring to choose from.
Take the spring you want to modify and stretch it out slightly. Give it an extra 30% length or so. Next compress the spring to see if it would shrink back any. Now trim off whatever extra length you added to the spring using the wire cutters. For a two spring button, you might want to trim off a little more than you added since the second (unmodified) spring would still give the tension needed to hold both the button and the contact plate. If not, stretch out the upper spring to lengthen it slightly. Reassemble the button and test it using a voltmeter to see how much force is needed to activate it. Repeat the above steps if you want to make the button more sensitive. Remember, you can always trim more off the spring later, but you can never go back if you trim off too much.