wiki/tools.nix.overrides-and-overlays.org

Nix overrides and overlays

You can override values in Nix as a way to customize nixpkgs. For example, if you want to use a different version from the nixpkgs channel, you can change the appropriate value.

let overlay = self: super:
      {
        ncmpcpp = super.ncmpcpp.override { visualizerSupport = true; };
      }

For another example, you can see some examples from Neovim and Veloren (which also uses Nix flakes).

You can set overlays automatically either by setting nixpkgs.overlays from your system configuration or ~/.config/nixpkgs/overlays/ folder for user-specific settings. You could also set overlays for standalone Nix code similarly through the overlays key — e.g., import <nixpkgs> ? { overlays = (self: super: { } ); };.