Hi
You can't - you only can switch to R1C1 style (column numbers are displayed)
and back to column labels.
When you want this for better lookout only (without any functionality), then
you always can set any number of top rows as header rows, and use Freeze
Panes feature to set them always visible on top.
When you want refer to them in formulas, then you can define them as Named
Ranges. An example:
Define named range TestNumber=Sheet1!$A:$A
Into any cell outside of column A enter the formula
=SUM(TestNumber)
All numeric data in column A are summarized.