It might be easier and better sounding to insert a sentence. But sentence or word, the procedure is the same. First, load the new word or sentence into the bin.
1. Right-click on the clip in the sequence that's to be changed and UNlink
2. Right-click the blank area to the left of the video track on the sequence and LOCK TRACK (If there are additional video tracks, lock them as well)
3. Position the cursor (the red line) at the start of the word or sentence to be replaced.
Right-click the very top of the cursor (looks like a downward arrow) and SPLIT ALL TRACKS
Do the same for the end of the word or sentence
4. Click the audio clip that was just created by those splits and press the <del> key on the keyboard to create a gap (or right-click | DELETE)
5. Reposition the cursor at the start of the gap (The ||< and >|| buttons below the preview window can help to do this accurately)
6. Click the audio tab above the bin, right-click the new audio clip and OVERLAY ON SEQUENCE AT CURSOR
This will place the new audio on its own track, below the gap, so as to not disturb synchronization
Play
If the word doesn't fit, re-record it and try again, as above, until it does
If you make a mistake along the way, press <ctrl-Z> to backtrack