Categories and Consolidation

Hey Everyone,

I am wondering if there is an easy way to quick edit categories like an excel spreadsheet. So if category A and category B is clicked on a post, then is there some way to have these posts also be categorized into category C?

Essentially I am looking to edit a massive list of posts and have a bunch of them be included within the new category. Any suggestions or ideas on how to do that? Or does it just require the long grind to do it?