update

The update subcommand is a simple alias for:

nix flake update

As it sounds, this will update your lock file.

Updating Package Sources

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