To use the Google Chat, simply open a new tab in your browser and go to chat.google.com. Or if you prefer a standalone app, you can download it here - https://chat.google.com/download/
Any group messages you may have created in Hangouts will no longer be visible in Chat, but new groups can easily be recreated as rooms. For more information about using Hangouts Chat, see our knowledge base article or visit this G Suite Learning Center webpage.