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).
7z u archive.zip *.doc
updates *.doc files to archive archive.zip.
7-Zip doesn't uses the system wildcard parser. 7-Zip doesn't follow the archaic rule by which *.* means any file. 7-Zip treats *.* as matching the name of any file that has an extension. To process all files, you must use a * wildcard.
Switches that can be used with this command
Switches: -u (Update)