What you’re looking for is a batch search and replace function. These are common in web design software (e.g., Dreamweaver) and in applications like Notepad or MS Word. Unfortunately, they are not as common in average blog applications. All is not lost, though. I found a few things that may help you.
WordPress offers a plug-in called APAD that allows you to search and replace text across all your posts.
Typepad users can find information from this help article in the Typepad Support Knowledge Base. Looks like you need the Pro level or above if you want to search and replace across multiple posts.
Blogger users may be out of luck, I’m sorry to say. I was unable to find a way to do a batch search and replace. That, of course, doesn’t mean there’s not a way, but I couldn’t find it if there is. Perhaps one of our readers will be so kind as to comment on this.