You don't say which version of Now Up-to-Date you're using, but if it's a version prior to Now X, you simply choose "Save as..." from the file menu and give your calendar file whatever name you like.

If you're using Now X, there is no file to save, the database is stored in your user library in the application support area, and the title of the widow "untitled" is actually the name of the active tab. If you want to rename a tab, simply double click on its name to select and type in whatever name you like.
Sean Boiarski
Product Manager, Now Software