To modify tables, cells, rows, and columns, you must first select them.
Sometimes you must select rather than just click a table to modify it. Selecting ensures that the table is in the correct state for the steps that follow.
Click the table, then click 
in its top-left corner; three white squares appear on the edges of the table when it’s selected.
Select a cell: Click it.
Select a cell to add or edit its content: Double-click it.
Select a range of cells: Click a cell, then drag a white dot any direction to encompass the range of cells you want.
If you drag the yellow dot, you copy the cell contents to the rows you drag over. If you do this accidentally, drag the yellow dot back to its original position, or press Command-Z on the keyboard to undo the action.
Select nonadjacent cells: Click a cell, then Command-click any other cells.
When you select a table cell, the Smart Cell View appears at the bottom of the Numbers window. The Smart Cell View shows you the actual value of a selected cell (for example, “Apr 3, 2014 12:00 AM”), or the formatted value of a cell if you’re editing it (for example, “4/3”).
If the selected cell contains a formula, the Smart Cell View shows you the formula. If you’re editing the formula, the Smart Cell View shows you the formula result.
You can also select a range of cells to see quick calculations for those cells—such as the sum, average, minimum value, and maximum value—in the Smart Cell View.
Click the table.
Letters appear above the columns and numbers appear to the left of the rows.
Do any of the following:
Select a single row or column: Click the number or letter for the row or column.
Select multiple rows or columns: To select adjacent rows or columns, click the number or letter for the first row or column, then drag a white dot to encompass the rows or columns you want.
Select nonadjacent rows or columns: Command-click any row numbers or column letters.