To override a package the package set, simply add a package with the same name to your dependencies.
let
prelude-override =
purs-nix.build
{ name = "prelude";
src.path = /home/me/my-prelude;
info = /package.nix;
};
in
purs-nix.purs
{ dependencies =
with purs-nix.ps-pkgs;
[ console
effect
prelude-override
];
}