Open WhatsApp
Tap the three dots or Settings
Go to Linked Devices if you want to log out of a connected device
Select the device
Tap Log Out
On iPhone, go to Settings, then Linked Devices, then select the device and log out
To log off the app on your phone, back up your chats first if needed
On Android, go to Settings, then Apps, then WhatsApp, then Force Stop or Clear Data
On iPhone, delete the WhatsApp app if you want to sign out completely
Reinstall WhatsApp and verify your number again when you want to use it later