The A, B, C headers are internal to Excel, and are used to specify cell
locations. They can't be changed to text.

However, if you need headers for a report or something, you can type
them in and make them bold, underline them, etc. If the lettering
system REALLY bothers you, you can turn it off (that is to say, remove
it from view on screen- they'll still be there) by clicking Tools
Options, selecting the View tab, and de-selecting the "Row and column headers" box.