You can do a search for the value in question, like "United States Of America" (in quotes) and it should find every tag with that value. Then, after selecting all of those files, you can change that tag to what you want (USA for example).
Another way to do this, which is more "sure" is to use a Panes view. Build a Panes view with some Panes. Make one of those Panes the the Tag you want to work on. In this case the Pane would be Country. Now you will see EVERY value that the Country tag has, including the ones you don't want. It's very easy with a Panes view to simple select the value you don't like, which will show every file with that value. Then just highlight those files, open the Tagging Window, and change the Tag value to what you want it to be.
These kinds of editing operations are part of the core power of MC which makes it so wonderful.
Good luck!
Brian.