How Do I re-record voice over?

You could highlight the selection in waveform view and hit record. You'll then start overwriting the highlighted portion of your waveform and it'll stop at the end of the highlight section. This is good if you can fit everything into the same times. You can also always undo if you start the recording and want the old version back.

Another way would be to record the new sentence and then copy it and past into the original waveform by highlighting the portion you wish to overwrite and pasting here.

Finally, if you're working in multitrack you may wish to take a look at punch and roll. It's amazing for doing the kind of thing you'd like but works in multitrack only.

You can't really do this in Waveform view, which seems like the obvious solution - if you hit 'record' at the point you want to insert, you'll erase what's already there.

The simplest way to do this is in Multitrack, but it's not quite as difficult as you think. First, re-record the sentence. If you find the point where you want to do the insert, put the cursor on it an right-click. You'll find a menu option that says 'silence' and you insert a bit more silence than you need to fit the file in. Put it where you want it, and then select the over-size gap where you allowed too much. Right-click and select Ripple Delete. This has a further option that says Gap. Click on that, and it's shrunk, with everything else moved up to it. Mix down the result - you're done.

This actually takes longer to describe than it does to do!