Select the cell with the formula
Move your cursor to the fill handle at the bottom-right corner of the cell
Click and drag the fill handle across the cells you want to fill
Release the mouse button to copy the formula
Use double-click on the fill handle to fill down automatically if adjacent cells have data
Use Ctrl+C and Ctrl+V to copy and paste the formula if needed
Use Ctrl+D to fill down selected cells
Use Ctrl+R to fill right selected cells