For questions regarding how to update the Pi's operating systems, programming languages, and applications (both native and external). Consider also tagging with the program or OS that you are updating if appropriate.
update is a command used with
apt, the package management system used on Raspbian and other Debian derived GNU/Linux distributions such as Ubuntu. This command is distinct from
man apt for more details).
This tag does not refer specifically either of these, but is appropriate for questions about their use; it may also be used on on questions that have nothing to do with
apt but do concern the general concept of updating or upgrading. Tag-wise here,
upgrade is a synonym for
update (i.e., they are the same).