Open the Facebook app or website
Go to Settings and privacy
Tap Settings
Open Password and security
Tap Where you’re logged in
Find the device using Messenger
Tap Log out
If needed, log out of Facebook to log out of Messenger on that device
On Android, you can also clear the Messenger app data from your device settings
On iPhone, you can uninstall and reinstall Messenger if you need to remove the session from the device