Kerstin here from the Shopify Guru Team :)
You should be able to delete any tags you don't need any more from your Products via the Bulk Editor. If you're still unable to delete them, please reach out to firstname.lastname@example.org, start a live chat or give us a call and we can dig deeper into this for you.
All the best,
Kerstin | Shopify Guru
One more option to bulk update your tags, is to use this app: https://apps.shopify.com/excel-export-import
It's allows you the Bulk Import Export Update with Excel file.
You basically export all your Products, then remove all columns form that sheet, which you don't want to update. Basically, you can keep only ID and Tags columns. Then set the values for Tags whatever you like, and import that file back. It will update only that one "Tags" field.