To change tags on a mass of posts can you use the default mass edit mode of WordPress. This more secure as a sql-statement.
If you are on the edit post page, there you see all posts. Open the options on right top and edit a number of posts, there you will edit, maybe 500.
After this changes switch to the mass edit mode. Active all posts with the checkbox and use Edit in the Bulk Mode. Also a screenshot.
Now you get a textarea with all tags, there are in all posts and you can remove the tags.
A alternative is the plugin Search Replace, that can change tags to each post via sql statement, but is more directly on the database and not secure.