My toddler likes to yell. Her instant reaction whenever she gets upset is to yell. How do I teach her to stop yelling? It's really frustrating that she does this, not to mention painful to our ears

Talk to her in the manner and tone that you would like from her. Its very normal that they tell as they do not know how to regulate their emotions. My son yells too. I think on the whole at this age they are just loud.