How to Use Goal Seek in Spreadsheet

1. Create a spreadsheet in Excel that contains your data.

2. Click the cell you want to change. This is called the “Set cell”.

3. From the Tools menu, select "Goal Seek"

4. In the Goal Seek dialog, enter the new “what if” amount in the To value text box.
(Remember to add the percentage sign if you have one.) 

5. Click OK. Excel will overwrite the previous cell value with the new one.

Sumber:
http://www.timeatlas.com/5_minute_tips/general/finding_answers_with_excel_goal_seek