I found this in Blogger Help. If you want to change the name of a label you have in your posts, read on....
Change the name of a label
Let's say you have a number of posts with the label Hawaii but you've decided that you'd rather call it Vacation instead. You can't edit the name of a label directly, but there's a simple workaround to accomplish your goal:
Go to the Posts tab for your blog.
Click on the "All labels" drop-down on top-right and select "Hawaii." This will filter the posts, so you'll only see those with the "Hawaii" label.
Click the check box at the top of the list of posts to select all the posts.
Then, click the Label icon and select "Hawaii" to remove the label "Hawaii" from all the posts that are selected.
While the posts are still selected, click the Label icon again and select "Add new label." Then, type in "Vacation."