Mine had a wire leading from the body to the bumper. The bumper end was held on by two small Phillips screws, which I undid to disconnect it (the bit it screws on to falls off the top of the bumper).

When putting it back together, make sure to put it on again the right way round, with the wire towards the front of the roomba (i.e. furthest from the main body), otherwise you will find the wire gets in the way of the bumper working, and you'll have to take it all apart again, like I did!