The update
subcommand is a simple alias for:
nix flake update
As it sounds, this will update your lock file.
If you pass directory name then it will update that input if the directory contains a flake.nix, with an optional arguement to update only a specific input in the subflake.
For example, you can update any package sources you may have declared in pkgs/flake.nix:
flk update pkgs
or just its nixpkgs:
flk update pkgs nixpkgs