u (Update) command
Update older files in the archive and add files that are not already in the archive.
Note: The current version of 7-Zip cannot change an archive which was created with the solid option switched on. To update a .7z archive you must create and update that archive only in non-solid mode (-ms=off switch).
Example
7z u archive.zip *.doc
updates *.doc files to archive archive.zip.
Switches that can be used with this command
-i (Include)
-m (Method)
-p (Set Password)
-r (Recurse)
-sfx (create SFX)
-si (use StdIn)
-so (use StdOut)
-ssw (Compress shared files)
-t (Type of archive)
-u (Update)
-w (Working Dir)
-x (Exclude)
See also
Commands: a (Add), d (Delete),
Switches: -u (Update)