If Excel is already open, you can open your spreadsheet by pressing Ctrl + O (Windows) or Cmd + O (macOS) and then selecting the file.
For example, to select the first column (column A), click the A at the top of the column. If you want to hide multiple columns at once, just click and drag your cursor over the column letters you want to hide. You can also select multiple non-adjacent columns by holding down Ctrl as you click each column letter.
If you don’t have multiple mouse buttons, hold down the Ctrl key as you click the column(s) instead.
