There is a quick way to select the data in all the rows and columns with just one click.
Find the blank rectangle above row number 1 and to the left of Column A.
Click that box and all cells are now selected and turn light blue.
Selecting all your data in one sheet is very useful. You can quickly copy or delete the data in the entire sheet, change the formatting such as the font style or alignment of all cells, or use the data to quickly add a pivot table.