all my top rows are now numbers, how to set it back to letters

[Holly]

My excel's top rows are numbers now, how to set it back to letters?

 

[Gary''s Student]

Pull-down:

Tools > Options... > General and clear the R1C1 checkbox

 

[PCLIVE]

Go to Tools-Options. Click the General tab. Uncheck R1C1 reference style.