Use the SUM or SUMPRODUCT function in Excel to quickly add up a range of numerical values. It's important to note that if it's only numerical values that need to be added up, use the SUM function. However, if your calculation involves multiple ranges of numerical values, use the SUMPRODUCT function.

First, select the cell you would like to begin summing the numerical values for the output.
Second, write out the SUM or SUMPRODUCT function you would like to use in the selected cell.
Third, input the numerical range within the parentheses of your function.
Fourth, press enter to view the output that calculates all the numerical values within the range you specified.
Finally, if you need to change something within the function, double click on the cell, make the necessary changes, and press enter on your keyboard. The output should automatically update.
In summary:

1. Select the cell you would like to begin entered the SUM/SUMPRODUCT function into
2. Write out the SUM or SUMPRODUCT function into the selected cell
3. Input the numerical range you would like to calculate within the parentheses of your function
4. Press enter to view the output
5. If changes need to be made, double click the cell, make the changes, and press enter