The SUM function in Excel is a handy tool for adding up values in a range of cells. Here's how to use it:

1. Enter the formula:

- In an empty cell where you want the sum displayed, type `=SUM(`.

2. Specify the range:

- Inside the parentheses, enter the cell range containing the numbers you want to add.
- You can directly click and drag to select the cells on the spreadsheet.
- Or, you can type the cell references separated by a colon (e.g., A1:A10 for cells A1 to A10).

3. Close the formula:

- After specifying the range, type a closing parenthesis `)` to complete the formula.

For example:

- If you want to add the values in cells A1 to A10, you would enter the formula `=SUM(A1:A10)`

Press Enter:

- Once you've entered the formula, press Enter on your keyboard. Excel will calculate the sum of the values in the specified range and display the result in the cell containing the formula.

Additional Notes:

- You can also include individual cell references or other formulas within the SUM function parentheses to add them to the range.
- For instance, `=SUM(A1:A10, B5)` would add cells A1 to A10 and cell B5.

- The SUM function can handle numbers, currency values, and even text that represents numbers. However, it will ignore text labels or blank cells.