Go to the "Chats" tab, select the necessary chats, then click on the pushpin icon option in the upper panel
Done! From now on, the selected chats will be displayed at the top of the list; you can only pin up to 3 chats
You can unpin chats in the same way by selecting them and clicking on the crossed out pushpin
Download WhatsApp and read the full article where you can find out other interesting WhatsApp tricks.