You can't change the column headings to text.
You can, however, make column headings of your own choosing in Row 1 and
then go to Tools--Options--View and deselect "show column headings" so row 1
would become your de facto column heading.
You would probably then want to freeze row 1 so that if you scrolled down it
wouldn't disappear. To do that, select row 2 and go to Window--Freeze panes.
Dave