I'm not sure if this is the best place to post this.

I'm attempting to create my first plugin. I'm trying to figure out how to change the health UI bar. I have already figured out how to set a higher max health, but the health bar will not exceed 100. What should I do to fix this? I can't seem to figure out how to alter default UI elements if it is even possible.

I'm aware of the plugin BetterHealth, but some of the players on my server are getting graphical errors from the way BetterHealth creates its own health bar. Thats why I'm trying to make my own plugin that hopefully is able to just change the default UI objects rather than create an entirely new UI object.

As 2CHEVSKII said, it's not possible. Have you considered scaling damage instead? Like this,

private void OnEntityTakeDamage(BasePlayer player, HitInfo info)
{
        info.damageTypes.ScaleAll(0.5f); //scales damage to 50%
        return;
}