How will you make modification in …

Click File > Options > Advanced.
-OR-
In Excel 2007 only: Click the Microsoft Office Button  , click Excel Options, and then click the Advanced category.
Under Editing options, do one of the following:
To enable Edit mode, select the Allow editing directly in cells check box.
To disable Edit mode, clear the Allow editing directly in cells check box.
Enter Edit mode
To start working in Edit mode, do one of the following:
Double-click the cell that contains the data that you want to edit.
This starts Edit mode and positions the cursor in the cell in the location that you double-clicked. The cell contents are also displayed in the formula bar.
Click the cell that contains the data that you want to edit, and then click anywhere in the formula bar.
This starts Edit mode and positions the cursor in the formula bar at the location that you clicked.
Click the cell that contains the data that you want to edit, and then press F2.
This starts Edit mode and positions the cursor at the end of the cell contents.