How can I make my metal byfold doors look better?

The easiest thing would likely be to paint them. Take them off and spray paint them with a paint meant for metal. :)

Paint them and then add a pretty design with a stencil.

Take them down and paint them-Rustoleum and Krylon are good products, stenciling is a good idea also to add to them. Dollar Tree has some pretty decals that might do the trick if you are interested in decorating them. You might go to several Dollar Trees, they vary in availability and design.

Removing them and spray painting them with a new coat of paint should make them look great again.

I agree with the other commenters, taking them down and painting them would be the quickest and easiest update. I would use Rustoleum spray paint for the best finish.

18 Makeover Closet Door Ideas That’ll Give You Closet Envy

https://www.onecrazyhouse.com/closet-door-makeovers/

you can take them down and spray paint them - paint them a fun color and once they dry rehang them

I would paint the doors, remove the slats and add fabric. https://www.exquisitelyunremarkable.com/2019/07/simple-shutter-makeover.html

You could try some alkyd paint which wouldn't have to be sprayed. Or if it's not too bad, maybe use some tape, vinyl, contact paper or peel and stick wallpaper to create a pattern or design.

You can definitely paint them using spray paint or gloss enamel paint.