You need to tell us why you want it as text. You have lots of other choices
of formats available.

If you absolutely have to have text, you will need to create a helper cell,
and use a formula like:
=text(a1,"m/d/yy")

Thanks for the suggestions. I'm saving this excel file as *.csv and
opening it in a GIS. But the GIS is having problems reading the
columns with dates and times (e.g. 30:26.4). I thought that a format
change could help, but maybe not.