In fact you cannot rename a label in blogger, but you have to remove it from all the post and add the new name. Check out the link below to do it in very simple steps
Change the name of a label – Blogger Help: 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:
Labels are the only way in Blogger to categorize a post. Here we cannot create sub-categories / labels like we have in other platforms like WordPress. Joomla, etc, but still Blogger is a wonderful free blogging platform by Google.