Select the track, open the Inspector (the scissors icon) and you will see the waveform. You can select sections of this (drag the mouse along it) but in order to edit it you need to split the track at the beginning and end of the section : the problem is that you need to replace a section with a bleep of exactly the same length. (You would actually find this easier in an audio editor such as Amadeus II:
http://www.hairersoft.com/Amadeus.html - $30)
Your best bet would be to select the offending word and copy it out, paste it into an empty new track, find a bleep sound and replace the word in this new track, being careful to keep the length the same, and then paste it back over the word (which you should still have selected) in the original track.
You could do something similar in Garageband, though it will be a lot clumsier because you can't just select then delete, you have to use the 'split' function..