Audacity doesn’t have punch in or out, but you can create a new track under the old one with the Overdubbing tools. As if you wanted to sing three part harmony or lay down four guitar tracks by yourself.
Play the old track in your headphones and sing along with it. Just keep singing when you get to the flub. Stop and crossfade, slide it back and forth, etc. between the two tracks until it sounds good. The problem with punch-in is it destroys the old track.