It's the app with a green chat bubble on your home screen or in the app drawer. Because WhatsApp has no built-in Log Out button, you'll have to sign out by deleting the app's data from your device. To make sure you don't lose your chats, back them up to your Google account. Here's how: Tap at the top-right of the screen. Tap Settings at the bottom of the drop-down menu. Tap Chats. Tap Chat Backup. Tap Backup. It's the round button at the bottom center area of the screen. This will bring you back to the home screen. It's the app with a gray gear icon on your home screen or in the app drawer. It's under the "Devices" header. The apps are listed in alphabetical order, so you may have to scroll down quite a bit. If you don't see a Storage option but see a button that says "Clear Data," just skip to the next step. If you see a confirmation message asking if you want to delete the app's settings and files, tap OK. Otherwise, just skip to the next step. It will open to the sign-in screen, indicating that you've been logged out. When you want to sign back in, open WhatsApp and enter your username and password. You'll be prompted to tap Restore to restore from the backup you made.

Summary:
Open WhatsApp. Back up your data. Press the Home button. Open your Android's Settings. Scroll down and tap Apps. Scroll down and tap WhatsApp. Tap Storage. Tap Clear Data. Open WhatsApp.