* {
  margin: 0;
  padding: 0;
}
.cnt {
  display: flex;
  margin: 50px;
  background: #ffff00;
  justify-content: space-between;
  flex-wrap: wrap;
}
.cnt div {
  width: 100px;
  height: 100px;
  background: #999;
  margin: 10px;
}
.cnt::after {
  content: '';
  width: 100%;
}
.flex-item:nth-child(6) {
  order: 1;
}<div class="cnt">
  <div class="flex-item one"></div>
  <div class="flex-item two"></div>
  <div class="flex-item three"></div>
  <div class="flex-item four"></div>
  <div class="flex-item five"></div>
  <div class="flex-item six"></div>
</div>The problem I have is that I can break line only on the last item... How am I actually able to brake line whenever I want it to? For example, if I want to break line after class .two, how can I do it?
EDIT
I'm an idiot. This is actually how you do it. You just need to pass the items you want to be in the next order. Obviously you may do this only once.
So, to break line in the second box, you need to pass the .three, .four, .five and .six to order: 1. See here: https://jsfiddle.net/r33muub3/4/ 
I changed the justify-content to left so it will be much more obvious
 
     
    