Can you change an alternator belt from under a 1998 Chevy Metro?

No, you generally cannot comfortably or safely change an alternator belt from underneath a 1998 Chevy Metro (or most cars, for that matter). The belt is usually located on the top of the engine, making access from above necessary. Working from underneath would severely restrict your visibility and maneuverability, making it a difficult and potentially dangerous job. You'll need to work from the top, likely requiring you to lift the hood.