Open WhatsApp
Tap the three dots or Settings
Go to Linked Devices if you want to log out from a connected device
Select the device
Tap Log out
On iPhone, go to WhatsApp Settings
Tap Linked Devices
Select the device
Tap Log Out
To log out from the app itself, back up your chats first if needed
Delete the WhatsApp app from your phone
Reinstalling and verifying your number will sign you back in