Category and tag URLs have bases that indicate whether they are for categories or tags. For example, the URL for a category named Cars looks like this:
…and the URL for a tag named Florida looks like this:
If you would like to change either of these bases, you can do so at:
Dashboard > Settings > Permalinks > Optional
Note that some plugins may disable category and tags bases; in that case, these settings will not have any effect.
If you need to swap your categories and tags, there’s an importer tool for that!
Dashboard > Tool > Available Tools > Cat/Tag Converter > Install Now