You can use the Flexbox. He works as a flex container, capable of sorting and directing the elements children. To use it, you need to determine display: flex; and direct according to the requirements you want, in your case flex-direction: column;, this configures the column ordering of your Ivs. By default, it always directs in line.
After you apply this setting to container, it is necessary to add to the elements children the parameter order, setting with numerical value the order you want to display your Divs (this also applies to negative values).
.container {
display: flex;
flex-direction: column;
}
.divum {
width: 100%;
height: 200px;
background: black;
order: 2;
}
.divdois {
width: 100%;
height: 200px;
background: red;
order: 1;
}
<div class="container">
<div class="divum"></div>
<div class="divdois"></div>
</div>
There are other definitions that you can apply depending on your need on the site origamid there is great content on the subject. You can also see on CSS-Tricks and in the MDN.
In case that div would be responsive, could not do something failed from this model
– Josimara