Carry the pointer of your mouse over the header you want to select.
And left click on it. See the diagram below.
Excel then selects the entire column of cells for you.
Tip: If you want to select the entire Worksheet (all the rows and columns), just press the ‘Ctrl’ + ‘A’ keys on your computer keyboard. To deselect it, press anywhere on your Worksheet.
Lesson 14: How to fill a Range with the same Data