If you have a large worksheet with formulas and you want to perform an autofill, then it may be a tedious job to drag the fill handle across hundreds of rows or columns. Here is a simple procedure to make your life easier:
1. Select the first cell in your range (the one which has a formula already).
2. While holding down the shift key go to the last cell in the range and select it. Now you should have all your range selected.
3.In the Edit menu click Fill then Series.
4. When you get the Series window click Autofill.
All cells in the range should now be updated with formulas.