Tag Managing Thing allows you to edit your tags.
http://downloads.wordpress.org/plugin/tag-managing-thing.zip has the latest version.
Drop the plugin file into your wordpress plugins folder.
Enable the plugin
In the Manage section of wp-admin, there is a Tags menu item. Selecting it will take you to the Tag Managing Thing interface.
It allows selecting the taxonomy to work with. Currently in wordpress there is a category and a post_tag taxonomy. Selecting a taxonomy allows editing the terms within that taxonomy.
Once a term has been selected you can perform the following tasks
- Change the name and slug of the term
- Delete a term
- Split a term into multiple terms
- Merge other terms into the selected term
- Change a term to belong in a different taxonomy