John, tags can be imported and exports in CSV files from searches, lists and filters. If you want to remove or add tags in batches, you can also do that from a list with a batch update. You cannot overwrite existing tags with an import, so you'll want to do that by deleting unwanted tags completely from the Tags menu, or removing them from a selected list using a batch update.

Here are some related resources:

How to import (be sure you have the right ID or email associated with each tagged list you're importing so that a merge occurs) 

What does overwrite existing data mean?

How do I import tags?

How to batch update