How do I Add a Label to an Existing post?

Well in blogger.com you can do this by clicking on the icon at the bottom of the post that looks like a pencil. That will bring you to the edit window. (Or from your dashboard you can click on: Edit Posts - and this will give you a list of the posts which you can edit)

Now that you're looking at the posting at the bottom of the page you'll see the: Labels for this post. Just add the labels that you want there. Click on show all if you want to include the post in a label/topic you've already created!

There ya go!

Sincerely,
CB Webmaster

P.S. Remember to click PUBLISH POST after you add the labels to save those labels.

P.S. You can edit the labels for multiple posts at once from the Edit menu - and don't worry about accidentally relabeling 10 files simultaneously incorrectly - they have an UNDO button! hehe