Summarize:

To highlight multiple cells, click the top left-most cell of the data, then drag your cursor down and to the right until all cells are highlighted. A menu will appear. The name of this option varies by version. It's either on the top or side of the window that popped up. It's on the side of the window. Click the down-arrow next to the “Decimal places” menu to display the list of numbers, then click the one you want to select.  Example: To round 16.47334 to 1 decimal place, select 1 from the menu. This would cause the value to be rounded to 16.5. Example: To round the number 846.19 to a whole number, select 0 from the menu.  This would cause the value to be rounded to 846. It's at the bottom of the window. The selected cells are now rounded to the selected decimal place.  To apply this setting to all values on the sheet (including those you add in the future), click anywhere on the sheet to remove the highlighting, and then click the Home tab at the top of Excel, click the drop-down menu on the “Number” panel, then select More Number Formats. Set the desired “Decimal places” value, then click OK to make it the default for the file. In some versions of Excel, you'll have to click the Format menu, then Cells, followed by the Number tab to find the “Decimal places” menu.
Enter your data series into your Excel spreadsheet. Highlight any cell(s) you want rounded. Right-click any highlighted cell. Click Number Format or Format Cells. Click the Number tab. Click Number from the category list. Select the number of decimal places you want to round to. Click OK.