Click Lock in the dropdown when clicking on the identicon icon:
You can also set an auto-lock, which you can find in Settings > Advanced:
Here you can set a timer in minutes after which the wallet will be locked.
You will see Log Out in the sidebar tapping it will prompt a confirmation message to log out:
Please note: you can also add an extra layer of security by adding a hardware wallet. Refer to this article on hardware wallet integration.