can I undo HDR in my phone?

Once the picture has been taken you can't undo that process (that's for Photo mode, Portrait you can alter after the fact), you could edit the image to your preference and then save the result though.

You can under Camera Settings select to save HDR and Normal, but that will only take affect for pictures taken from now on.

Hello PortiaI,

Thanks for using the Apple Support Communities!

I see from your post that you are wanting to turn off the HDR setting of your Camera so that you'll have normal pictures.

Take a look at this section of the iOS 12 Guide called HDR camera on iPhone, which explains in detail how to turn off automatic HDR.

I hope this information helps you.

Kindest Regards.