You can do a Master delete of all occurrences using the Tag Browser. Right click the tag name.
Just remember, it is DUMB. If a book only has that single tag, it is still GONE.
I recommend using this method to clear the BS/Garbage tags first.
You can also use the Tag Browser to adjust the CASE of a tag. Right-click:change case...
You can also fix spelling or tense. Series: Mystery vs Mysteries (
I choose singular as it applies to the Book, as my standard. I also get rid of junk metadata as applied to a Title or Series:
A Novel,
Series when used in the title)