Use the flex-direction Property to Make a Row

Summary

The flex-direction property can be used to align the children of an element into rows or columns.
Other options for flex-direction are row-reverse and column-reverse.

Final Code

<style>
  #box-container {
    display: flex;
    height: 500px;
    flex-direction: row-reverse;
  }
  #box-1 {
    background-color: dodgerblue;
    width: 50%;
    height: 50%;
  }

  #box-2 {
    background-color: orangered;
    width: 50%;
    height: 50%;
  }
</style>

<div id="box-container">
  <div id="box-1"></div>
  <div id="box-2"></div>
</div>