archive-7z-update is a variable defined in `arc-mode.el'.
Its value is
("7z" "u")

Program and its options to run in order to update a 7z file member.
Options should ensure that specified directory will be put into the 7z
file. Archive and member name will be added.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.