Command U - update files within an archive
This command, as well as command f, updates files in the archive only if they are older than files being added, but unlike "f", it will add new files as well.
This is the command line equivalent of the command Add to archive with "Add and update files" update mode selected in the Archive name and parameters dialog.
Because archive volume modification is disabled, this command is not applicable to volumes.
See also: command A - add files to archive