The most simple way to bleep out a word is to select the word then “Generate > Tone” over the top of it.
A more tasteful way is to use some sort of effect to make the offensive word unintelligible, such as this “scratch” effect: https://forum.audacityteam.org/t/scratching-effect/20435/2
Instructions for installing plug-ins: http://audacityteam.org/download/plugins