Google Chat has replaced Hangouts in Gmail. During the transition, you can access Hangouts at hangouts.google.com.

What You Should Do

1. If you use the mobile app, uninstall Hangouts, and install Google Chat for Android or iOS.

• If you do not use any chat features, you don't need to do anything.

2. For group messages, create new groups in Chat or use the rooms feature. 

• Learn about the differences between group messages and rooms.

Directions and Tutorials

How to use Google Chat in Gmail.

Google Chat Help Site