Open WhatsApp
Tap the three dots menu on Android or Settings on iPhone
Go to Linked Devices if you want to sign out from a specific device
Select the linked device
Tap Log out
On Android, to sign out of the app itself, go to Settings on your phone
Tap Apps
Tap WhatsApp
Tap Force stop
Tap Storage
Tap Clear data or Clear storage
On iPhone, to sign out of the app itself, delete the app from the device
Reinstall WhatsApp if you want to use it again later