One of the most useful tools in Microsoft Word is the Find & Replace feature, which makes modifying existing documents or updating names or locations a breeze.  To launch the interface, press Ctrl-H, then simply tell Word which word(s) you want it to find in the Find what: box and type the text you want to replace it with in the Replace with: box. 

If you want to make sure that your selection was correct, you can click on the Replace button and each instance will come up one at a time.  If you are confident in your instructions, you can click on Replace All and viola!