Do you know them all?
You can select the contents:
- Press Ctrl + A
- Select Edit then Select All from the menu (older versions of Excel)
- Click the gray square in the top-left corner of the spreadsheet (above the number 1 and to the left of the letter A)
Why select the entire spreadsheet? You might need to copy to another spreadsheet. Or want to adjust the formatting such as font and size. Or you might want to adjust column or row size by clicking on the column/row dividers.
Note: When you Crtl+A in a cell with adjoining cells that have content, the region will be selected first. To select the entire spreadsheet, just Ctrl+A again.