'u' - UPDATE FILES IN ARCHIVE(S)

The update command updates one or more files in one or more archives, although
typically it is used to update files in only one archive.  The updating
process adds files which are not yet in the archive, and replaces older files
which already exist in the archive.  When replacing files, the 'last
modification dates' of the files in the archive are compared with those of
their counterparts specified at the command line, and the earlier modification
date is assumed to represent the older file.

As with the add command, only the file name component of the file is stored
by default.  The -x or -r options should be used to preserve path names.

Two commands similar to update are the freshen and replace commands.  The
freshen command is almost identical to the update command, except that no new
files will be added to the archive; only existing files in the archive may be
updated, depending on their last modification date.  The replace command is
almost identical to the freshen command, except that all specified files are
always updated, regardless of their last modification date.