It's simply "Upgrade the Database" or maybe "When the new version is installed, there must be a way to migrate the existing database". If you already know more details about this step, then include them. But the story mostly exists to make sure something isn't forgotten; it's not to be detailed.
Later, when you get to implement this story, you can flesh it out (which tables, do we need one or more backups, is there a fall back scenario, etc).
OTOH, if the project is more complex, this can become a "tag", like a post-it notice which must be attached to many stories. That means you must include this as a "sub story" to all stories which change the database. As you can see, these "project-spanning stories" are a bit hard to track with agile methods.
