Copy the word
Paste it
Underline it
Give it custom animation such as 'appear' 'after previous', either 'On
click' or after some time
Move it exactly over the original word.
I would not recommend doing this if the presentation will be
distributed to other people. The text in the original text frame could
easily be spaced differently due to font or platform differences on a
different computer, and then the overlapping text will not be in the
same place.