How to change CSS property inside a descendant selector

How do I change or remove style “width” property for img?

HTML:

<div class="main_box_img">
    <img src="...">
</div>

CSS:

.main_box_img img{
    width: 100%;
}

document.getElementsByClassName("main_box_img")[0].getElementsByTagName('img')[0].style is empty.

I can get width value using getComputedStyle(), but it’s read-only.

Seems to be a simple question but I’ve found nothing.