Inserting defined values/change and update

Hello,

I was wondering if Excel has an tool that will allow you to insert an specific number that you defined earlier. You insert that number eg. 8 time in your woorksheet. Then your boss says that you have to change that value. Then, insted of searching each value and manualy re-entering new value, you just redefine your custom number and update. Its something like inserting filed in MS Word.
Can this be done in Excel?
Thanx!