Find and Replace is a simple Google Documents function that has a ton of utility when you're re-using a document and only have to make minor adjustments to it. I was asked about this earlier today so I decided to make a couple of screenshots to demonstrate the process.
Step 1: Open the Edit drop-down menu and select Find and Replace.
Click image to view full size.
Step 2: Complete the Find and Replace With fields then click replace or replace all.