How do you customize or change your health bar over your head?

I kinda get how to script and stuff, but I just need to get a more feel of it and understand functions, I'm not sure if this is a dumb question or not, but any help is appreciated

You can't actually modify the build-in Health indicator.

The Humanoid in your character enables that Health bar, although it is possible to hide it by setting Humanoid.HealthDisplayDistance to 0.

In order to replace the Health bar entirely, you'll have to make a custom one using GUI.

If you have any questions about using GUI, feel free to ask here!