There is no menu for renaming the labels in Blogspot, but you can use a simple trick so that the label will change to the new name.
For example I will change the old label named KOMPUTER to become new label named ELEKTRONIKA (I use my language for label names).
1. Select for filtering the label name that you want to change. I choose KOMPUTER. All posts with label: KOMPUTER will show.
2. Select all posts by giving a tick on them (of course all posts with label: KOMPUTER)
3. Click the Label icon then select New label.
4. Write new label name (ELEKTRONIKA)
You will see all posts with label: KOMPUTER also has label: ELEKTRONIKA