Using order property as shown in below code :- HTML:-

<div id="main">
  <div style="background-color:coral;" id="myRedDIV"></div>
  <div style="background-color:lightblue;" id="myBlueDIV"></div>
  <div style="background-color:lightgreen;" id="myGreenDIV"></div>
  <div style="background-color:pink;" id="myPinkDIV"></div>
</div>

CSS:-

#main {
  width: 400px;
  height: 150px;
  border: 1px solid #c3c3c3;
  display: -webkit-flex; /* Safari */
  display: flex;
}


#main div {
  width: 70px;
  height: 70px;
}


/* Safari 6.1+ */
div#myRedDIV   {-webkit-order: 2;}
div#myBlueDIV  {-webkit-order: 4;}
div#myGreenDIV {-webkit-order: 1;}
div#myPinkDIV  {-webkit-order: 3;}


/* Standard syntax */
div#myRedDIV   {order: 2;}
div#myBlueDIV  {order: 4;}
div#myGreenDIV {order: 1;}
div#myPinkDIV  {order: 3;}