Tap on three dot icon from top right corner and select Settings
Tap on SMS and MMS
Tap on SMS Disabled
Readers must note that this option of using Signal as your default SMS/MMS app is only available to Android users. “In addition to supporting end-to-end encrypted messages and calls with other Signal users, Signal Android can be configured as your default SMS/MMS app,” says Signal in its support pages. The option to do this is not available if you the platform on iOS and desktop.
When an Android user makes Signal their default SMS/MMS app, the contact list shows both Signal contacts and non-Signal contacts and a Blue letter in the outside column will indicate it is a Signal contact.