Open WhatsApp
Tap the three dots on Android or Settings on iPhone
Go to Linked Devices if you want to log out from a connected device
Select the device
Tap Log out
If you want to log out from your phone, back up your chats first if needed
Uninstall WhatsApp from your phone
Or clear app data on Android from Settings > Apps > WhatsApp > Storage > Clear data
On iPhone, delete the app to remove access from the device