if you're just trying to change how the hud elements look, you can replace those like you would any other graphic. you'll find which filename corresponds to which graphic inside "data/graphics/hardcoded".
if you're trying to move things around or add custom elements, that's a bit more complicated and requires at least some knowledge of lunalua scripting.
here's a thread that goes into hud design a bit more specifically, and in case you're unfamiliar with lunalua, here are some related links that might be useful: